Satoshi Kanazawa

The Scientific Fundamentalist

Why are mothers better parents than fathers? Part II

How does the 'biological clock' make mothers better parents?

Posted Jun 15, 2008

The best they can do is better for men

In my last post, I explained that paternity uncertainty -- the distinct possibility of cuckoldry -- is one reason that fathers are less committed to their children than mother are. The other reason is the absence for men of the proverbial “biological clock.”

The second biological factor that makes fathers less committed parents is their higher fitness ceiling (the best they can do reproductively). Fetuses gestate for nine months within the female body, and infants are, at least in the past, nursed by the mother for several years after birth, during which the mother is usually infertile (lactational amenorrhea). Women also have a much shorter reproductive life than men do due to menopause. These two factors combine to create a much higher fitness ceiling for men than for women.

Men can potentially have many more children than women can. The largest number of children that a man has ever had is at least 1,042. The last Sharifian emperor of Morocco, Moulay Ismail the Bloodthirsty (pictured above), maintained a large harem, as many ancient rulers did, and had at least 700 sons and 342 daughters. The exact number of children that Moulay Ismail had in his lifetime is lost to history, because they stopped counting them after a while. The reason the recorded number of sons is more than twice as many as the recorded number of daughters is because they stopped counting daughters long before they stopped counting sons.

In contrast, the largest number of children that a woman has ever had is 69. The wife of an eighteenth-century Russian peasant, Feodor Vassilyev, had 27 pregnancies in her life, including 16 pairs of twins, seven sets of triplets, and four sets of quadruplets; amazingly, Mrs. Vassilyeva never had any single births in her life! And all but two of her 69 children survived to adulthood.

Exactly how many children Moulay Ismail the Bloodthirsty and Mrs. Vassilyeva had is not important. What’s important is this: The largest number of children that a man can potentially have is two orders of magnitude greater than the potential number of children that a woman can have (thousands vs. tens).

The massive sex difference in the largest possible number of children means that, while reproductive success is equally important to men and women (in fact, to all biological organisms), each child is far more important to the mother than it is to the father. Each child represents a far greater proportion of a woman’s lifetime reproductive potential than it does a man’s. If a 40-year-old mother of five deserts her children and they die as a result, she will likely end her life as a total reproductive loser, having failed to leave any copy of her genes in the next generation. If a 40-year-old father of five does the same, he can go on to produce five (or ten or twenty) more children.

Both paternity uncertainty and the higher fitness ceiling make fathers less committed parents than mothers, and this is why there are so many more deadbeat dads than deadbeat moms; very few women abandon or neglect their children. Ironically, it is the mother’s greater commitment to her children that allows the father to neglect them even more. Knowing the mother’s greater commitment to her children, the father can abandon them, secure in the knowledge that the mother would never do likewise, because if she did, the children would be virtually certain to die. In other words, divorced parents with children are playing a game of Chicken, and it is usually the mother who swerves. Most fathers would probably prefer to invest in their children and raise them by themselves rather than see them die, but they normally do not have to make this difficult decision, because they know that the mother would never abandon them. The mother’s greater commitment to her children ironically allows the father to have his cake and eat it too, by moving on to the next marriage and family in which to invest.

I will conclude this series with the next post, in which I discuss yet another example of “the exception which proves the rule.”

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