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It doesn’t make sense that exercise is good for you.  Its just unnecessary wear and tear on your body. Take the analogy of a car.  Would it make sense to take it out for a drive up and down the block just to “exercise” it?  Your car will survive for only so many miles and you are wasting them with exercise.

But exercise is supposed to pay off in the long run. Sure you are wasting resources and subjecting your body to potential injury by exercising but if you survive the exercise you will be stronger as a result. Still this is hard to understand. Because its your own body that is making itself stronger. Your body is re-allocating resources away from some other use in order to build muscles. If that’s such a good thing to do why doesn’t your body just do it anyway? Why do you first have to weaken yourself and risk injury before your body begrudgingly does this thing that it should have done in the first place?

It must be an agency issue. Your body can either invest resources in making you stronger or use them for something else. The problem for your body is knowing which to do, i.e. when the environment is such that the investment will pay off. The physiological processes evolved over too long and old a time frame for them to be well-tuned to the minute changes in the environment that determine when the investment is a good one.  Your body needs a credible signal.

Physical exercise is that signal.  Before people started doing it for fun, more physical activity meant that your body was in a demanding environment and therefore one in which the rewards from a stronger body are greater. So the body optimally responds to increased exercise by making itself stronger.

Under this theory, people who jog or cycle or play sports just to “stay fit” are actually making themselves less healthy overall. True they get stronger bodies but this comes at the expense of something else and also entails risk. The diversion of resources and increased risk are worth it only when the exercise signals real value from physical fitness.


I haven’t decided yet and I can’t figure out which side this is evidence for:

Me:  Oh I have to remember to set up your desk today because I promised that I would and that if I didn’t I would give you $2.

7 year old:  I was hoping you would forget.

Me:  Are you saying you would rather have $2 than your desk?

7yo:  No, I am saying I would rather have $2 today and my desk tomorrow.

Me:  Hold on, what would you rather have:  $2 today and your desk tomorrow or $2 today, another $2 tomorrow and then your desk the next day?

7yo:   $2 today, another $2 tomorrow and then my desk the next day.

Me:  $2 today, $2 tomorrow, $2 the next day, and then your desk the day after that?

7yo:  Yep.

Me:  And what is the number of days you would like to have $2 before you finally get your desk?

7yo:  Infinity.

From The New Yorker

Now, imagine an animal that emerges every twelve years, like a cicada. According to the paleontologist Stephen J. Gould, in his essay “Of Bamboo, Cicadas, and the Economy of Adam Smith,” these kind of boom-and-bust population cycles can be devastating to creatures with a long development phase. Since most predators have a two-to-ten-year population cycle, the twelve-year cicadas would be a feast for any predator with a two-, three-, four-, or six-year cycle. By this reasoning, any cicada with a development span that is easily divisible by the smaller numbers of a predator’s population cycle is vulnerable.

Prime numbers, however, can only be divided by themselves and one; they cannot be evenly divided into smaller integers. Cicadas that emerge at prime-numbered year intervals, like the seventeen-year Brood II set to swarm the East Coast, would find themselves relatively immune to predator population cycles, since it is mathematically unlikely for a short-cycled predator to exist on the same cycle. In Gould’s example, a cicada that emerges every seventeen years and has a predator with a five-year life cycle will only face a peak predator population once every eighty-five (5 x 17) years, giving it an enormous advantage over less well-adapted cicadas.


We were interviewed by the excellent Jessica Love for Kellogg Insight.  Its about 12 minutes long.  Here’s one bit I liked:

We go around in our lives and we collect information about what we should do, what we should believe and really all that matters after we collect that information is the beliefs that we derive from them and its hard to keep track of all the things we learn in our lives and most of them are irrelevant once we have accounted for them in our beliefs, the particular pieces of information we can forget as long as we remember what beliefs we should have.  And so a lot of times what we are left with after this is done are beliefs that we feel very strongly about and someone comes and interrogates us about what’s the basis of our beliefs and we can’t really explain it and we probably can’t convince them and they say, well you have these irrational beliefs.  But its really just an optimization that we’re doing, collecting information, forming our beliefs and then saving our precious memory by discarding all of the details.

I wish I could formalize that.

Single-Origin Shame

The other day I heard this chef talking on the radio about dropping lobsters into boiling water. The question was whether this or any other method of cooking live lobster was humane. Specifically he was focusing on the question of whether the lobster feels pain.  The chef’s preferred method was to first put the lobster in the freezer until it stops moving and then drop it into boiling water.

Of course there is no way to know whether the lobster feels pain from being boiled alive but we can ask whether there is any theoretical reason it would feel pain.  In creatures that feel it, pain is a selected response to a condition in the environment that is to be avoided. Notice an implication of this:  being a (life-)threatening is a necessary but not sufficient condition for some environmental feature to induce the response of pain.

Apparently humans do not feel pain, or anything at all, when exposed to life-threatening carbon monoxide.  Presumably that is because relative to the span of time it takes to evolve a protective painful response, carbon monoxide has not been a relevant threat for very long.  No response has been selected for yet.

Does a lobster ever encounter hot water in its natural environment?  Is there any channel through which natural selection would have given lobsters a painful response to being boiled?  What about being frozen?

Cuckoos lay their eggs in other birds’ nests and when the cuckoo chick is born it kicks all of the eggs out of the nest and monopolizes the care and attention of the cuckolded parent.  Fairy wrens have evidently evolved a countermeasure:

Diane Colombelli-Negrel from Flinders University in Australia has shown that mothers sing a special tune to their eggs before they’ve hatched. This “incubation call” contains a special note that acts like a familial password. The embryonic chicks learn it, and when they hatch, they incorporate it into their begging calls. Horsfield’s bronze-cuckoos lay their eggs too late in the breeding cycle for their chicks to pick up the same notes. They can’t learn the password in time, and their identities can be rumbled.

Which is incredibly cool.  An ingenious solution and a testament to the resourcefulness of the evolutionary invisible hand.  Especially when you notice that the cuckoo chick “is a huge, grey monster that looks completely unlike a warbler chick.”  Apparently evolution favors the complex system of teaching and repeating a singing password rather than the boring solution of just staring at the invader and noticing that he looks nothing like a cute little fairy wren.

I remember a commercial for some kind of diet program where that was the tagline.  A disembodied hand kept enticing this poor guy with delicious looking food and then taking it away because it was unhealthy and then the voiceover came in with that line and I thought that was so tragic that everything that tastes good had to be bad for you.  Like what kind of cruel joke is that?

And it makes no sense from a biological point of view.  I should want to eat what’s good for me so that I do eat what’s good for me and avoid what’s bad for me. That’s Mother Nature’s optimal incentive scheme.  And once we have evolved to the point that we can think and understand that principle we should be able to infer that whatever tastes good must in fact be good for us.  But it’s not!

At the margin it’s not.    Indeed the right statement is “If it tastes good then you surely have already had too much of it to the point that any more of it is bad for you.” Because the basic elements in food that we love, namely sugar, salt, and fat, are all not just good for us but pretty much essential for survival.  And so of course we are programmed to like those things enough that we are incentivized to consume enough of them to survive.

But the decision whether to eat something is based on costs as well as benefits. Nature programmed our tastes so that we internalize the benefits but it’s up to us to figure out the costs:  how abundant is it, how hard is it to acquire, and when it’s sitting there before us how likely is it that we will have a chance to eat it again in the near future.  Then we need to weigh the costs and benefits and eat up to the quantity where marginal costs equal marginal benefits.

It’s interesting that Nature put a little price theory to use when she worked all this out.  A price is a linear incentive scheme.  Every additional unit you buy costs you the same price as the last one.  Your taste for food is like a linear subsidy, every unit tastes about as good as the last, at least up to a point. When you face linear incentives like that you consume up to the point where your personal, idiosyncratic marginal cost equals the given marginal benefit.  If a planner (like your Mother Nature) wants to get you to equate marginal cost and marginal benefit, a (negative) price is a crude incentive scheme because the true marginal benefits might be varying with quantity but the subsidy makes you act as if its constant.

But that’s ok when the price is set right.  The planner just sets the subsidy equal to the marginal benefit at the optimal quantity.  Then when you choose that quantity you will in fact be equating marginal cost to the true marginal benefit.  That’s a basic pillar of price theory.

So Nature assumed she knew pretty well what the optimal quantity of sugar, salt, and fat are and gave us a taste for those elements that was commensurate with the true marginal benefit at that optimal point.  And its pretty much a linear incentive scheme at least in a large neighborhood of the target quantity.  Sugar, salt and fat don’t seem to diminish in appeal until we have had quite a lot of it.

The problem is that the optimal quantity depends on both the value function and the cost function.  Now the value function, i.e. the health benefits of various consumption levels is probably the same as it has always been.  But the cost function has changed a lot.  Nature was never expecting Mountain Dew, Potato Chips and Ice Cream. The reduction in marginal cost means that the optimal quantity is higher, but how much higher?  That depends on how the shape of the value function at higher quantities.  The old linear incentive scheme contains no information at all about that.

But one thing is for sure.  If the true marginal benefit is declining, then at higher quantities the linear incentive scheme built into our taste buds overstates the marginal benefit.  So when we equate the new marginal cost to the linear price we are doing what is privately optimal for us but what is certainly too high compared to Nature’s optimum.  If it tastes good its bad for us we because we have already had too much.

You must watch Balasz Szentes’ talk at the Becker Friedman Institute. At the very least, watch up until about 7:00. You will not regret it. (Note that Gary Becker was sitting in the front row.)

Because of runners’ high:

When people exercise aerobically, their bodies can actually make drugs — cannabinoids, the same kind of chemicals in marijuana. Raichlen wondered if other distance-running animals also produced those drugs. If so, maybe runner’s high is not some peculiar thing with humans. Maybe it’s an evolutionary payoff for doing something hard and painful, that also helps them survive better, be healthier, hunt better or have more offspring.

So he put dogs — also distance runners — on a treadmill. Also ferrets, but ferrets are not long-distance runners. The dogs produced the drug, but the ferrets did not. Says Raichlen: “It suggests some level of aerobic exercise was encouraged by natural selection, and it may be fairly deep in our evolutionary roots.”

The story is from NPR, the pointer is from Balazs Szentes.

Dawkins couldn’t be more dull when he is playing the heretic.  When he is excoriating heretics, on the other hand, he is sharp as a tack:

Misunderstanding Number One, which is also perpetrated by Wilson, is the fallacy that “Kin selection is a special, complex kind of natural selection, to be invoked only when the allegedly more parsimonious ‘standard Darwinian theory’ proves inadequate.” I hope I have made it clear that kin selection is logically entailed by standard Darwinian theory, even if the B and C terms work out in such a way that collateral kin are not cared for in practice. Natural selection without kin selection would be like Euclid without Pythagoras. Wilson is, in effect, striding around with a ruler, measuring triangles to see whether Pythagoras got it right. Kin selection was always logically implied by the neo-Darwinian synthesis. It just needed somebody to point it out—Hamilton did it.

Edward Wilson has made important discoveries of his own. His place in history is assured, and so is Hamilton’s. Please do read Wilson’s earlier books, including the monumental The Ants, written jointly with Bert Hölldobler (yet another world expert who will have no truck with group selection). As for the book under review, the theoretical errors I have explained are important, pervasive, and integral to its thesis in a way that renders it impossible to recommend. To borrow from Dorothy Parker, this is not a book to be tossed lightly aside. It should be thrown with great force. And sincere regret.

Check this out. Five numbers appear on a screen in different locations. They remain visible for 210 milliseconds and then they are obscured. The subject must then touch the locations in increasing order of the numbers that appeared there. That’s pretty much impossible. Here’s a human subject who is highly trained and does an impressive job but still fails miserably.


Now check out how nonchalantly this chimpanzee does it.


I didn’t even know they could count. Note that the 5 numbers are random integers between 1 and 9.  So the chimp is processing a binary relation in short-term memory, not to mention reading at a super-human rate. There are more videos here.  I saw these at Colin Camerer’s talk last week at Arthur Robson‘s conference on the Biological Basis of Preferences.

Over the weekend I attended a conference at the University of Chicago on The Biological Basis of Preferences and Behavior, and Balazs Szentes stole the show with a new theory of the peacock’s tail.  In Balazs’ theory a world without large and colorful peacock plumage is simply not stable.

A large tail is an evolutionary disadvantage:  it serves no useful purpose and it slows down the male and makes him conspicuous to predators.  So why do genes for large tails appear and take over the population of male peacocks? Balazs’ answer is based on matching frictions in the peacock mating market. Suppose female peacocks choose which type of male peacock to mate with: small or large tails. Once the females sort themselves across these two separate markets, the peacocks are matched and they mate.

The female peacocks are differentiated by health, and within a peacock couple health partially compensates for the disadvantageous tail. In the model this means that healthy females who mate with big-tailed peacocks will produce almost as many surviving offspring as they would if they mated with peacocks without the disadvantage of the tail.

This substitution between the characteristics of female and male peacocks creates a selection effect in the mating market. Consider what happens when a small-tailed peacock population is invaded  by a mutation which gives some male peacocks large tails. Since female peacocks make up half the population of peacocks there is now an imbalance in the market for small-tailed peacocks. In particular the males are in excess demand and some females will have trouble finding a mate.

On the other hand the big-tailed male peacocks are there for the taking and its going to be the healthy female peacocks who will have the greatest incentive to switch to the market for big tail. The small cost they pay in terms of reduced quantity of offspring will be offset by their increased chance of mating. The big tails have successfully invaded.

Once they have taken over the population (Balasz shows that under his conditions there is no equilibrium with two kinds of male peacocks) he same selection effect prevents small tails from invading. When a small-tail mutation appears all the females will want to mate with them. The market for small tail gets flooded with eager females up to the point where some of them are going to have a hard time finding a mate. Given this, each female must decide whether to take a gamble and try to mate with the small-tail male or have a sure chance of mating with a large tail.

The unhealthy females are going to be the ones who are most willing to take the risk because they are the least compatible with the large-tail males. This means that the small-tail mutants can only mate with unhealthy females and (under the conditions Balazs identifies) this more than offsets their advantage, they produce fewer offspring than the large-tails and they are driven out of the population.

Eye color and cuckoldry:

The human eye color blue reflects a simple, predictable, and reliable genetic mechanism of inheritance. Blue-eyed individuals represent a unique condition, as in their case there is always direct concordance between the genotype and phenotype. On the other hand, heterozygous brown-eyed individuals carry an allele that is not concor- dant with the observed eye color. Hence, eye color can provide a highly visible and salient cue to the child’s heredity. If men choose women with characteristics that promote the assurance of paternity, then blue-eyed men should prefer and feel more attracted towards women with blue eyes.

This calls for an experiment.

The eye color in the photographs of each model was manipulated so that a same face would be shown with either the natural eye color (e.g., blue) or with the other color (e.g., brown). Both blue-eyed and brown-eyed female participants showed no difference in their attractiveness ratings for male models of either eye color. Similarly, brown-eyed men showed no preference for either blue-eyed or brown-eyed female models. However, blue-eyed men rated as more attractive the blue-eyed women than the brown-eyed ones. We interpret the latter preference in terms of specific mate selective choice of blue-eyed men, reflecting strategies for reducing paternity uncertainty.

Acquired traits passed on to descendants:

“In our study, roundworms that developed resistance to a virus were able to pass along that immunity to their progeny for many consecutive generations,” reported lead author Oded Rechavi, PhD, associate research scientist in biochemistry and  at CUMC. “The immunity was transferred in the form of small viral-silencing agents called viRNAs, working independently of the organism’s genome.”

An interesting theoretical explanation:

According to the CUMC researchers, Lamarckian inheritance may provide adaptive advantages to an animal. “Sometimes, it is beneficial for an organism to not have a gene expressed,” explained Dr. Hobert. “The classic, Darwinian way this occurs is through a mutation, so that the gene is silenced either in every cell or in specific cell types in subsequent generations. While this is obviously happening a lot, one can envision scenarios in which it may be more advantageous for an organism to hold onto that gene and pass on the ability to silence the gene only when challenged with a specific threat. Our study demonstrates that this can be done in a completely new way: through the transmission of extrachromosomal information. The beauty of this approach is that it’s reversible.”

Beanie bow:  Courtney Conklin Knapp

Suppose our minds have a hot state and a cool state.  In the cool state we are rational and make calculated tradeoffs between immediate rewards and payoffs that require investment of time and effort.  But when the hot state takes over we abandon deliberation and just react on instinct.

The hot state is there because there are circumstances where the stakes are too high and our calculations too slow or imperfect.  You are being attacked, the food in front of you smells funky, that bridge looks unstable.  No matter how confident your cool head might be, the hot state grabs the wheel and forces you to do the safe thing.

Suppose all of that is true.  What does that mean when a situation looks borderline and you see that instincts haven’t taken over?  Your cool, calculating head rationally infers that this must be a safer situation than it would otherwise appear.  And you are therefore inclined to take more risks.

But then the hot state better step in on those borderline situations to stop you from taking those excessive risks.  Except that now the borderline has moved a little bit toward the safe end.  Now when the hot state doesn’t take over it means its even more safe, etc.

And of course there is the mirror image of this problem where the hot state takes over to make sure you take an urgent risk.  A potential mate is in front of you but the encounter has questionable implications for the future.  Physical attraction receives a multiplier.  If it is not overwhelming then all of the warning signs are magnified.

“Corporations are evil” and we know this because they are always doing malicious things that are only later exposed. This often involves exploiting the complexity of transactions and the inability or unwillingness of consumers to wade through the thicket by surreptitiously ripping people off.  For example, unauthorized charges inserted into phone bills, in a practice known as “cramming”, cost Americans $2 billion dollars a year, according to this article.

When something like this is discovered, the automatic reaction is to assume that the malice was intentional. They were sticking those charges in there to squeeze money out of consumers. And its basic economics that if they can secretly insert charges and make money they will. On the other hand, such a theory would appear to require you to accept they hypothesis that “corporations are evil” or at least they are cold-hearted profit maximizers.

But you can believe that corporations are not intentionally malicious and still assume that whenever there is a cold-hearted way to steal money they will do it.    Because many malicious practices are not actively designed, rather they creep in and they are passively allowed to persist.

For example, those charges could have been legitimate under an outdated policy and when the policy was changed they forgot to remove them. Or some bumbling technician could have accidentally inserted them. Modern transactions are so complicated that random “mutations” are going to appear without any malicious intent and indeed without anyone noticing. This is a far more likely explanation than someone purposefully sticking them in there, especially if you doubt that “corporations are evil.”

Indeed, to have a conscious policy of ripping off unsuspecting customers requires instructing somebody to do that, and leaving a paper trail. Even a truly evil corporation understands that this is the wrong way to do it. The right way to do it is to structure the organization in a way that facilitates malice creep.

You don’t have to instruct anybody to allow mutant ripoffs to appear. They appear on their own, no paper trail required. All you need to do is to give weak incentives to the officers you have charged with making sure that you are not ripping anybody off.  Nobody in your organization will have any knowledge of all the ways you are cheating your clients, not even you. By design.

There is an art to the design of an organization that cultivates malice creep. Because at the same time you have to stop “virtue creep” in its tracks. You don’t want unintended credits to randomly get inserted into the phone bill. What you need is a one-sided monitoring program. You wait around for lots of mutations to appear, you know that some are virtuous and some are malicious. Now getting rid of the virtuous ones and keep the malicious ones is easily done, just announce that its time to do some “cost-cutting.” Form an ad hoc task force to go through and find ways to restructure billing in ways that save the company money. They’ll just look at the credits and ignore the charges.

In terms of the long-run bottom line, Darwinism and Lamarckism are almost indistinguishable, but Occam’s razor favors Darwin.  I would argue by the same principles that most of the malicious practices of organizations emerge by cultivated accident rather than by design.


Here’s what you already know:  it’s a parasite that reproduces in the digestive system of cats.  The eggs are excreted out and the vehicle is consumed by other animals in whose brains the eggs develop. Only when those brains are consumed by other cats does the cycle continue.  In order to facilitate this process, chemicals are secreted inside the hosts’ brain to make them do things to increase the chance they will be eaten by cats.  For example, rats with toxoplasma in their brains are not afraid of cats.

Here’s what’s new:  toxoplasma is transferred from host to host through sexual contact in order to get closer to cats.

These are Toxoplasma cysts moving from rat to rat, so this exchange is kind of like a side track on the parasite’s life cycle. But it still benefits Toxoplasma, because it means it can infect even more potential prey that may get eaten by cats. And so the logic applies once more: if Toxoplasma can raise the odds of getting from infected males to uninfected females, it may have more reproductive success.

You know where this is going–it’s turning into a David Cronenberg horror movie with an all-rodent cast. Vyas wondered if there’s any difference in how female rats mate with infected and uninfected males. So he and his colleagues put a male rat with Toxoplasma at one end of a two-armed maze, and an uninfected male in the other arm. Females then got to choose which rat to approach. Vyans found that they preferred the infected males, spending more time with them and mating more often.

It’s pretty old, but worth reading given his new book.

Trivers has been teaching himself things and then growing bored with them his whole life. In 1956, when he was 13 and living in Berlin (his father was posted there by the State Department), he taught himself all of calculus in about three months. Around the same time, and with more modest success, Trivers-a skinny child picked on by bullies-tried to learn how to box, doing push-ups and covertly reading Joe Louis’s ”How to Box” in the school library.

Akubra Cadabra: Tobias Schmidt.

If you think about pain as an incentive mechanism to stop you from hurting yourself there are some properties that would follow from that.

When I was pierced by a stingray, the pain was outrageous. The puncture went deep into my foot and that of course hurts but the real pain came from the venom-laden sheath that is left behind when the barb is removed. Funny thing about the venom is that it is protein based and it can be neutralized by denaturing the protein, essentially changing its structure by “cooking” it as you would a raw egg.

How do you cook the venom when it is inside your foot? You don’t pee on it unless you are making a joke on a sitcom (and that’s a jellyfish anyway.) What you do is plunge your foot is scalding hot water raising the internal temperature enough to denature the venom inside. Here’s what happens when you do that. Immediately you feel dramatic relief from the pain. But not long after that you begin to notice that your foot is submerged in scalding hot water and that is bloody painful.

So you take it out. Then you feel the nerve-numbing pain from the venom return to the fore. Back in. Relief, burning hot water, back out. Etc. Over and over again until you have cooked all the venom and you are done. In all about 4 hours of soaking.

A good incentive scheme is reference-dependent. There’s no absolute zero. Zero is whatever baseline you are currently at and rewards/penalties incentivize improvement relative to the baseline. When the venom was the most dangerous thing, the scalding hot water was painless. Once the danger from the venom was reduced, the hot water became the focus of pain. And back and forth.

Second Observation.  After three weeks of surfing (minus a couple of days robbed by my stingray friend) I came away with a sore shoulder.  Rotator cuff injuries are common among surfers, especially over the hill surfers who don’t exercise enough the other 11 months of the year.  The interesting thing about a rotator cuff injury is that the pain is felt in the upper shoulder, not at the site of the injury which is more in the area of the shoulder blade.  It’s referred pain.

In a moral hazard framework the principal decides which signals to use to trigger rewards and penalties.  Direct signals of success or failure are not necessarily the optimal ones to use because success and failure can happen by accident too.  The optimal signal is the one that is most informative that the agent took the appropriate effort.  Referred pain must be based on a similar principle.  Rotator cuff injuries occur because of poor alignment in the shoulder resulting in an inefficient mix of muscles doing the work.  Even though its the rotator cuff that is injured, the use of the upper shoulder is a strong signal that you are going to worsen the injury.  It may be optimal to penalize that directly rather than associate the pain with the underlying injury.

(Drawing:  Scale Up Machine Fail, from

If you are programming a robot to vacuum your floors here’s one thing you would never consider doing:  endow the robot with feelings of happiness and sadness and teach it to be happy when the floor is clean and unhappy when it is dirty.

But evolution led us to a state of affairs where emotions are what motivate us to do our jobs.  How could such a kludge arrive.

Here is a story.  Primitive organisms are reproduction machines.  They need a certain chemical in the environment, and when they can obtain that fuel they can reproduce.

So the most successful primitive organisms are those that are the best at finding fuel.  Natural selection favors those that seek fuel.

Next the organisms get more complicated.  They have to make decisions that involve more than just immediate reproduction.  They have intertemporal tradeoffs, multi-dimensional consumption, etc.

There is infrastructure in place to simplify this transition.  The organisms that have survived to this stage are the organisms that seek fuel.  They have built and learned systems for doing what is necessary to get fuel.  So fuel is a simple and effective incentive mechanism.

The organism could evolve a mechanism for storing and later releasing fuel. Fuel is released when the organism takes certain actions.  Fuel-seeking organisms will take those actions.  Natural selection will favor the organisms that release fuel for the right actions.

Now remember that fuel is the energy needed for the most primitive functions of the organism.  When this fuel is released the organism gets a boost of that energy.

A boost of energy is a big part of what we call happiness.

(subject to the usual disclaimer, this is based on some conversations with Balasz Szentes.)

Normally we understand the (near) 50-50 male/female population sex ratio with this simple model: if there were more males than females then individuals who are genetically disposed to have female children will have more grandchildren because their female children will find more mates. Thus females will increase in proportion, restoring the balance.

But here’s the interesting thing. That model doesn’t work for humans (and many other species) and in fact 50-50 is highly unstable, with potentially catastrophic consequences.

Suppose that a male has a mutation on his Y chromosome which causes him to produce Y-chormosome sperm that swim faster than his X-chromosome sperm. Then he will have only boys. And his boys will have the same gene and the same super Y-chromosome sperm.

Now suppose that his male children have an equal chance of mating as all other males in the population. Then our original mutant will have more male grandchildren than other males of his generation. Thus, the proportion of this super-Y gene increases in the population, and this trend continues generation after generation.

The balance is not being restored anymore. In fact eventually the super-Y’s dominate the male population. And that means that all offspring in all matings are boys. That means very little reproduction can happen because there are so few females. And the species goes exctinct.

I learned this from a paper by W. D. Hamilton called Extraordinary Sex Ratios.

It is a challenge for evolutionary theory to explain the prevalence of sexually reproducing species.  That’s because of the twofold cost of sex:  a sexually reproducing species produces half as many offspring per generation as an asexually reproducing population of the same size.  So not only must there be some other advantage to sexual reproduction, it has to be large enough to outweigh that substantial cost.

One theory is that the genetic mixing that comes from sex allows a species to shed disadvantageous mutations.  An asexual species can only accumulate them.  This advantage can be large enough to overcome the twofold cost of sex.  But the problem with this theoretical explanation is that in these models the advantage of sex is too large, so large that the kind of sex we see universally among all sexually reproducing species, sex between two parents, is dominated by tri-parental sex.  This was shown by Perry, Reny, and Robson who consider a particular kind of menage a trois in which each mating requires two males and one female, and each offspring receives half of its genetic material from the mother and one quarter from each of the fathers.

This avoids a tri-fold cost of sex:

Because the cost of males is determined not by the ratio of males to females in each mating instance but, rather, by the population ratio of males to females, de-termining the population ratio is central. We therefore turn to Fisher’s celebrated equilibrium argument (Fisher, 1930). Applying the same logic to 1/2 – 1/4 – 1/4 sex, we note first that the total reproductive value of all of the males in any generation is precisely equal to that of all of the females in that generation. This is because, un-der 1/2-1/4-1/4 sex, all of the females supply half of the genes of all future generations. But then the remaining half must be supplied by all of the males. Consequently, as Fisher argued, equilibrium requires the offspring sex ratio to equate parental expenditure on male and female offspring. Maintaining the usual assumption that offspring of either sex are equally costly to raise to maturity, we conclude that the equilibrium sex ratio must be one–each male therefore mates with two females and vice versa. But this means that the cost of males is twofold–there is no additional cost of males over biparental sex.

I bring this up because (in an older working paper version) they also considered the leading competing theory for the advantage of sexual reproduction, The Red Queen hypothesis.  Here the argument is that species are constantly trying to out-evolve parasites.  Genetic mixing makes them a moving target.  Perry, Reny, and Robson showed that, unlike the deleterious mutations theory, the Red Queen story rationalizes biparental sex over other forms of sex.  Thus, from the point of view of sex as an evolved mechanism for solving some problem, only the Red Queen can explain the kind of sex we see.

And I bring that up because just last week I heard this story about a new experiment that validates the Red Queen hypothesis.

Tyler Cowen quotes Richard Dawkins:

Isn’t it plausible that a clever species such as our own might need less pain, precisely because we are capable of intelligently working out what is good for us, and what damaging events we should avoid? Isn’t it plausible that an unintelligent species might need a massive wallop of pain, to drive home a lesson that we can learn with less powerful inducement?

There is an alternative to pain as an incentive mechanism:  dispensing with incentives altogether and just programming the organism with instructions to follow. And if the organism doesn’t already have “feelings” as a part of its infrastructure then the instructions are the only alternative.  The big question for theories of pain and pleasure as an incentive mechanism is why mother nature as Principal bothers with incentives at all.

I spent the weekend in bed with the flu.  Sunday morning, on the tail end of it, I popped a few Advil to bring the fever down so I could semi-enjoy Father’s Day. Was I making a mistake?

As I understand it, my body elevates its temperature as a defense mechanism. Evolution has been operating long enough to have a pretty well-calibrated trade-off between the losses of reduced activity from the fever versus the speed and probability of a successful recovery.  Is my intervention distorting away from the optimum?

  1. Arguably I have private information about idiosyncratic conditions and Nature is calibrated only to the average state.  Note that while this hypothesis justifies my use of Advil on Father’s Day, it also implies that I should go short on Advil on other days.
  2. And anyway Nature has given me the infrastructure to condition physiology on my knowledge of immediate environmental conditions.  For example when I know that I am in danger, the body re-allocates resources to help me escape.  What makes this any different?
  3. My objective is probably different.  In Mother Nature’s eyes I am just a vessel from which offspring should spring forth.  She could care less whether I get to practice Pink Floyd’s San Tropez on the piano with my daughters.  So Nature’s revealed preference for activity is necessarily weaker than mine.
  4. But wait, my personal preference for non-reproductive activity is also something that Nature shaped.  So what would explain the wedge?
  5. If I am making the wrong decision by taking Advil it’s not because I have the wrong preferences but because Advil is something Nature never expected.  She has me well-trained when it comes to the fundamentals but she hasn’t had time to design my direct preference for the intermediate good Advil. She must leave it up to me to do the calculation of its implied tradeoffs in terms of the fundamentals. It’s only because of my miscalculation that I am making a mistake.

Clearly the reason that sex is so pleasurable is because that motivates us to have a lot of it.  It is evolutionarily advantageous to desire the things that make us more fit. Sex feels good, we seek that feeling, we have a lot of sex, we reproduce more.

But that is not the only way to get motivated.  It is also advantageous to derive pleasure directly from having children.  We see children, we sense the joy we would derive from our own children and we are motivated to do what’s necessary to produce them, even if we had no particular desire for the intermediate act of sex.

And certainly both sources of motivation operate on us, but in different proportions. So it is interesting to ask what determines the optimal mix of these incentives. One alternative is to reward an intermediate act which has no direct effect on fitness but can, subject to idiosyncratic conditions together with randomness, produce a successful outcome which directly increases fitness.   Sex is such an act. The other alternative is to confer rewards upon a successful outcome (or penalties for a failure.)  That would mean programming us with a desire and love for children.

The tradeoff can be understood using standard intuitions from incentive theory. The rewards are designed to motivate us to take the right action at the right time. The drawback of rewarding only the final outcome is that it may be too noisy a signal of whether he acted.  For example, not every encounter results in offspring. If so, then a more efficient use of rewards to motivate an act of sex is to make sex directly pleasurable. But the drawback of rewarding sex directly is that whether it is desirable to have sex right now depends on how likely it is to produce valuable offspring.  If we are made to care only about the (value of) offspring we are more likely to make the right decision under the right circumstances.

Now these balance out differently for males than for females. Because when the female becomes pregnant and gives birth that is a very strong signal that she had sex at an opportune time but conveys noisier information about him.That is because, of course, this child could belong to any one of her (potentially numerous) mates. Instilling a love for children is therefore a relatively more effective incentive instrument for her than for him.

As for love of sex, note that the evolutionary value of offspring is different for males than for females because females have a significant opportunity cost given that they get pregnant with one mate at a time. This means that the circumstances are nearly always right for males to have sex, but much more rarely so for females. It is therefore efficient for males to derive greater pleasure from sex.

(It is a testament to my steadfastness as a theorist that I stand firmly by the logic of this argument despite the fact that, at least in my personal experience, females derive immense pleasure from sex.)

Drawing:  Misread Trajectory from

She wrote this convincing essay on happiness and parenting.  Parents seem to be less happy but we shouldn’t read too much into that.  She brings together all kinds of economic theory and data and along the way she cites a paper I like very much by Luis Rayo and Gary Becker:

Nobel Prize–winning economist Gary Becker, writing with Luis Rayo, has argued this contrary position. In their view, while “happiness and life satisfaction may be related to utility, they are no more measures of utility than are other dimensions of well-being, such as health or consumption of material goods.”[5] Or having kids. Children may make you less happy, but still raise your utility. Devout neoclassical reasoning leads Becker and Rayo to infer from the fact that we are having kids that they raise your utility (or at least they raise the utility of those who make this choice).

Rayo and Becker argued that happiness should be thought of as the carrot that gets us to make good decisions.  But happiness is a scarce resource.  There’s a limit to how happy you can be.  So it has to be used in the most economical way.  In their theory the most economical way to use happiness is to give an immediate, and completely transitory boost of happiness to reward good outcomes.  You have sex, you get rewarded.  It results in conception, that’s another reward.  But then you are back to the baseline so as to maximize the range available for further rewards (and penalties) motivating behavior going forward.  Bygones are bygones.

With that theory it makes no sense to look at a cross section of the population, compare how happy are people who did X relative to people who didn’t do X, and conclude on the basis of that whether its good to do X.

And by the way, if there is anything we can expect evolutionary incentives to have a good handle on, its whether or not to have kids.  That’s the whole ballgame.  If happiness is there to motivate us to succeed evolutionarily then you better have  a good argument why Nature got it wrong.  One place to look might be on the quantity/quality tradeoff.  Perhaps the relative price of quality versus quantity has declined in modern times and Nature’s mechanism is tuned to an obsolete tradeoff. If so, then people feel a motivation to have more kids than they should.  The prescription then would be to resist the temptation you feel to have another kid and instead invest more in the ones you have.  Unless you want to be happy.

Apparently it’s biology and economics week for me because after Andrew Caplin finishes his fantastic series of lectures here at NU tomorrow, I am off to LA for this conference at USC on Biology, Neuroscience, and Economic Modeling.

Today Andrew was talking about the empirical foundations of dopamine as a reward system.  Along the way he reminded us of an important finding about how dopamine actually works in the brain.  It’s not what you would have guessed.  If you take a monkey and do a Pavlovian experiment where you ring a bell and then later give him some goodies, the dopamine neurons fire not when the actual payoff comes, but instead when the bell rings.  Interestingly, when you ring the bell and then don’t come through with the goods there is a dip in dopamine activity that seems to be associated with the letdown.

The theory is that dopamine responds to changes in expectations about payoffs, and not directly to the realization of those payoffs.  This raises a very interesting theoretical question:  why would that be Nature’s most convenient way to incentivize us?  Think of Nature as the principal, you are the agent.  You have decision-making authority because you know what choices are available and Nature gives you dopamine bonuses to guide you to good decisions.  Can you come up with the right set of constraints on this moral hazard problem under which the optimal contract uses immediate rewards for the expectation of a good outcome rather than rewards that come later when the outcome actually obtains?

Here’s my lame first try, based on discount factors.  Depending on your idiosyncratic circumstances your survival probability fluctuates, and this changes how much you discount the expectation of future rewards.  Evolution can’t react to these changes.  But if Nature is going to use future rewards to motivate your behavior today she is going to have to calibrate the magnitude of those incentive payments to your discount factor.  The fluctuations in your discount factor make this prone to error. Immediate payments are better because they don’t require Nature to make any guesses about discounting.

  1. There is an inverse relationship between how carefully you stack the dishes inside the dishwasher and how tidy you keep it outside in your kitchen.
  2. In addition to funny-haha and funny-strange there is a third category of joke where the impetus for laughter is that the comedian has made some embarrassing fact that is privately true for all of us into common knowledge.
  3. It would be too much of an accident for 50-50 genetic mixing to be evolutionarily optimal.  So to compensate we must have a programmed taste either for mates who are similar to us or who are different.
  4. It is well known that in a moderately sized group of total strangers the probability is about 50% that two of them will have the same birthday.  But when that group happens to be at a restaurant the probability is virtually 1.

Following up on the Trivers-Willard hypothesis.  The evidence is apparently that promiscuity, a trait that confers more reproductive advantage on males than females, is predictive of a greater than 50% probability of male offspring.  A commenter claimed that there is a bias in favor of male offspring when the mother is impregnated close to ovulation and wondered whether the study controlled for that.  A second commenter pointed out that there is no reason to control for that because that may be exactly the channel through which the Trivers-Willard effect works.

So now put yourself in the shoes of the intelligent designer. Suppose you are given that promiscuity is such a trait.  You are given control over the male-female proportion of offspring and you are designing the female of the species.  What you want to do is program her to have male offspring when she mates with a promiscuous male.  But you cannot micromanage because there is no way to condition this directly on the promiscuity of the mate.  The best you can do is vary the sex proportions conditional on biological signals, for example the date in the cycle.

How would you do this?  Of all the “states of the system” that you can condition on, you would find the one such that conditional on having sex in that state, the relative likelihood that her partner was the promiscuous type was maximized.  You would program her to increase the proportion of male offspring in those states.

Is sex close to ovulation such a signal?  I don’t see why.  But we could think of some that would qualify.  How about the signal that he is delivering a small quantity of sperm?  The encounter lasted longer than usual, this is the first time she had sex in a while, these sperm have not been seen before, etc…

According to the Trivers-Willard Hypothesis, individuals possessing a trait which improves the reproductive success of males more than females will be more likely to give birth to male offspring than to female offspring.  I came across a study that claims to support the hypothesis where the trait in question is promiscuity.

Our analyses of two large nationally representative samples, from the General Social Survey in 1994 and the National Longitudinal Study of Adolescent Health, confirm this prediction.  Controlling for a large number of social demographic factors that might be expected independently to influence offspring sex ratios, unrestricted sociosexual orientation significantly increases the odds that the first child is a boy.  One standard deviation increase in the unrestrictedness of sociosexual orientation increases the odds of having a son by 12-19%.

That seems like a large effect, if true.  Chullo chuck:  Barking Up The Wrong Tree.

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