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…
Cheap Talk 
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February 10, 2011 at 11:39 pm
Doug
It’s thought that women are more likely to have extra-pair copulations during their ovulatory period (they are also more sexually interested and active generally, and there’s a report that women even show more skin at discos during this time). Assuming one believes all this, it’s not a huge leap to assume that extra-pair matings are more likely to take place with males who are generally more promiscuous. Say, the extra-pair mates might be chosen from open markets.
February 11, 2011 at 6:17 am
jeff
Makes sense.
February 11, 2011 at 2:42 pm
Dan
I tried to leave a comment saying something similar to Doug but it must’ve gotten eaten by the spam filter.
Women in a long-term relationship can have an extra-pair coupling in order to have offspring with a man who has better genes. The best time for her to do this is when she is when she is ovulating, since the ratio of risk (getting caught and losing her long-term partner) to reward (having a child with the good genes of her short-term partner) is at its most favorable when the probability of conception is highest. As Doug says, there is evidence that women’s behavior does vary across the ovulatory cycle in line with these predictions (if you google “behavioral effects of ovulation” the first result discusses some of this research; I won’t leave the link since I don’t want this comment to get eaten too).
That means that sex close to ovulation is more likely to be with a promiscuous man with good genes (and possibly with other genes that are particularly beneficial to male offspring).
February 11, 2011 at 11:00 pm
jeff
thanks! (i don’t know what happened to your original comment. the wordpress spam filter tends does tend to be aggressive sometimes.)
February 11, 2011 at 6:25 am
jeff
Further thoughts. The sperm of course determines the sex of the offspring so to some extent the sex selection could be controlled by having promiscuous males increase the proportion of sperm carrying Y’s. The question is whether the male internalizes all of the incentive to produce males over females. The child shares only half of his genes which suggests that he might not.
February 11, 2011 at 7:41 am
conchis
Why should we expect evolution to select the *optimal* signal? It should surely just select for any mutation that confesr *sufficient* evolutionary advantage.
February 12, 2011 at 10:44 am
Simon
I recall a study that ovulating strippers received bigger tips. This suggests that the (extra-marital) demand for women is higher at this point, in addition to the supply (ie increased promiscuity). Maybe this is the only time of the month they can get that cute guy in the office (you know, with the attractive genes).