Why Mothers Prefer Boys

Phil Kitcher is a philosopher who is interested in the philosophy of science and he's also very interested in evolution. In his recent article on The Trouble with Scientism he gives and example of .... well, I'm not exactly sure what.

The emphasis on generality inspires scientific imperialism, conjuring a vision of a completely unified future science, encapsulated in a “theory of everything.” Organisms are aggregates of cells, cells are dynamic molecular systems, the molecules are composed of atoms, which in their turn decompose into fermions and bosons (or maybe into quarks or even strings). From these facts it is tempting to infer that all phenomena—including human actions and interaction—can “in principle” be understood ultimately in the language of physics, although for the moment we might settle for biology or neuroscience. This is a great temptation. We should resist it. Even if a process is constituted by the movements of a large number of constituent parts, this does not mean that it can be adequately explained by tracing those motions.

A tale from the history of human biology brings out the point. John Arbuthnot, an eighteenth-century British physician, noted a fact that greatly surprised him. Studying the registry of births in London between 1629 and 1710, he found that all of the years he reviewed showed a preponderance of male births: in his terms, each year was a “male year.” If you were a mad devotee of mechanistic analysis, you might think of explaining this—“in principle”—by tracing the motions of individual cells, first sperm and eggs, then parts of growing embryos, and showing how the maleness of each year was produced. But there is a better explanation, one that shows the record to be no accident. Evolutionary theory predicts that for many, but not all, species, the equilibrium sex-ratio will be 1:1 at sexual maturity. If it deviates, natural selection will favor the underrepresented sex: if boys are less common, invest in sons and you are likely to have more grandchildren. This means that if one sex is more likely to die before reaching reproductive age, more of that sex will have to be produced to start with. Since human males are the weaker sex—that is, they are more likely to die between birth and puberty—reproduction is biased in their favor.

In humans, the average sex ratio at birth is about 105 boys to every 100 girls but this ratio varies a lot from country to country and it depends on environmental conditions. There are many factors that affect fertilization and the survival of embryos and fetuses.

Is it reasonable to believe that the observed sex ratio (1.05) is the product of natural selection? You can't really answer that question until you know the mechanism of altered sex ratios. What is being selected? Is it the probability that a male sperm will reach the egg before a female sperm? If so, what kind of selective advantage would have to apply to change that probability from from 50% to 51% or 52%? How is it done? What alleles are involved?

Why does Philip Kitcher, a philosopher of science, think that a postulated adaptive explanation is a "better explanation" than a mechanistic one? Don't you actually have to "prove" your adaptive model at the level of genes, cells, and developing embryos before it can be accepted?

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