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…