The Intelligence Paradox: Why the Intelligent Choice Isn't Always the Smart One - Satoshi Kanazawa 2012

Heritability of Fertility: An Evolutionary Puzzle
Why Intelligent People Are the Ultimate Losers in Life

Among both men and women, number of siblings significantly increases the number of children. Since the number of siblings (plus one) is the same as the number of children that their parents had, this means that fertility—the total number of children individuals have—is highly heritable. The more children your parents have had (and hence the more siblings you have), the more children you have yourself.

While this is consistent with earlier studies,⁴ heritability of fertility—where the number of children the parents had is positively associated with the number of children the children have—is a mystery from an evolutionary psychological perspective.⁵ The evolutionary psychological logic suggests that the association should be negative, not positive.

As I mention earlier in this chapter, there are two principal means for you to increase your reproductive success: you can have and raise your own children, or you can invest in close genetic relatives, such as your full siblings, who share half of your genes. But the option of maximizing reproductive success by investing in siblings is available only to those who have a large number of siblings. If you do not have any siblings, this option is not available to you. So you must have more children yourself if you do not have many siblings.

In contrast, if you do have a large number of siblings, then you don't necessarily have to have many children yourself, because you can increase your reproductive success by investing in your siblings. It is important to remember that you share just as many genes with your full siblings as you do with your own biological children. Both share half of your genes. Genetically speaking, it is slightly more advantageous to invest in your own children than in your full siblings, because your children belong in the next generation, not in the current generation with you and your siblings, and your children are expected to outlive your younger siblings. Nevertheless, it is possible to increase your reproductive success by investing in your siblings.

In Chapter 3, I discuss a fundamental principle of quantitative genetics: that there is an inverse relationship between the heritability of a trait and its adaptiveness; the more adaptive a trait, the less heritable it is.⁶ Fertility—the desire for and tendency to have children—is very adaptive; in fact, it's the very definition of reproductive success. So fertility should not be heritable at all, and all humans should be designed to have the maximum number of children that they can safely raise to sexual maturity, regardless of how many siblings they have.

If the number of siblings does affect fertility at all, then, for reasons I state above, evolutionary logic suggests that people who have more siblings should have fewer children (and instead invest in their siblings), and people who have fewer siblings should have more children. So the number of siblings and the number of children should be negatively correlated. Yet all the evidence suggests that this is not the case. Fertility appears to be heritable, and people who have more siblings have more children themselves. This remains an evolutionary puzzle.