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"Some guys have all the luck. Some guys have all the pain."
-- Song lyrics
Last time we saw thaton many traits, males are generally more variable than are females. Here is another example of males showing more within-sex variability than females: males generally have higher "reproductive variance" than do females. That is, there are more males at the extreme ends of the distribution of reproductive success (RS). Let's see how and why this is true.
Sex differences in reproductive variance
Assume that the sex ratio is 50/50 -- 50% males and 50% females (generally it is pretty close to that for people in their twenties), and that polygyny is illegal (yes, Vernon, it is illegal in the U.S.) . So, there should be an equal number of married men and women, right?
But look at the tables below of the percentages of people in the U.S. that have never been married. There is something puzzling in the data.
Percent of never married people. (U.S. Census)
In each census, for all age groups, there are always more never married men than women. How could this be? If each marriage involves one woman and one man, it should be impossible for there to be more never married men than never married women. What is up here?
Blue dots: More single men than women (age 18 – 34)
Also, take a look at this map. Each blue dot represents a city where there are more single males than females for people between the ages of 18 - 35. The one red dot is Springfield, MA (take note, fellas) where there are more single women than men in this age range. (Check out this link if you would like to mess around with an interactive version of the map.) Again, if the sex ratio is 50/50, how can there be more single men than women?
To answer these questions, let's take a look at what evolutionary biologists and psychologists call reproductive success -- the number of offspring an individual has in his/her lifetime. From an evolutionary perspective, higher reproductive success is better. Any genes that help to produce physical or psychological adaptations that generally increase reproductive success will survive and spread in the gene pool.
What is the average reproductive success of women? Of men? It has to be the same number for both sexes. But, the sexes can, and do, differ in the amount of within-sex variation in reproductive success.
What is the range of reproductive success for women? For our discussion here, let's assume that all women are fertile and want to have children. What is the maximum number of offspring a woman could realistically have in her lifetime? Assume a woman can have one child per year, and she has a fertility time window of 25 years. That would be 25 children on the high end of the reproductive success distribution (disregarding the possibility of having twins, triplets, etc.). On the low end of the distribution, at least for women of prime reproductive age (say, about 17 - 28) there are very few who, if they wished, could not find at least one man who might be willing to accommodate her sexual invitations. So the range of variance among women in number of children would be 1 - 25. Most women, of course, would bunch up near the mean of the reproductive success distribution, say, 2 - 3 children in the U.S. today.
What is the range of reproductive success for men? On the high end, what is the maximum number of offspring a man could theoretically have in his lifetime? Because men can leave the gestation to women, the number of offspring that they have is not limited as much by their own bodies. It is basically limited by the number of fertile female bodies to which they can gain sexual access. If we brought a man several fertile and willing women every day and he impregnated one per day, 365 days a year, over a reproductive lifetime of, say 50 years, that would be 18,250 children. The actual maximum on record is held by Moulay Ismael the Bloodthirsty, the emperor of Morocco from 1672 to 1727, who had 888 children. Some emperors of China may have had even more.
Now for the low end of the male distribution let's get brutally honest. This isn't going to be pretty. Are there some men who are just so unappealing (unattractive, stupid, poor, in poor health, and apparently with lousy genes) that no woman would want to have sex with him? No woman would choose that man to be the father of her children? Imagine a homeless man, empty whiskey bottle in hand, sleeping in a urine stained cardboard box. Any takers?
So the range of reproductive success for men is zero to, well, a very large number. And there are two groups of males for which there are no female equivalents: males who are "reproductively disenfranchised" at the low end of the distribution, and males who win a "reproductive jackpot" at the high end.
Joseph F. Smith family
The male reproductive "jackpot winners" were either polygynists (such as Joseph Smith -- see the photo), or, they got around to a lot of ladies (think Casanova). Whatever physical or psychological adaptations these men had that helped them to achieve such a high level of reproductive success were likely passed on to their male progeny.
Now, for the other category of men for which there is no female equivalent: reproductively disenfranchised men.
Homeless man in Paris
What percentage of men fall into this category? I sometimes ask this question to my classes, and I get answers from young women that range from 5% to 10% (and up!) of the male population. I don't know what the actual percentage might be. However, it is a sad fact -- there are indeed some men, regardless of their age, with whom no woman would want to risk a pregnancy. These guys are not happy campers. They are losing out in the most important biological game of life.
Now, back to the apparent anomalies of more unmarried men than women in the tables above, and the map that shows that most all cities in the U.S. have more single men than women (for the age range of 18 - 35).
These statistics now make sense because there is greater reproductive variance among men than among women. The male "reproductive jackpot winners" have many women, via multiple relationships and serial marriages. These men are "monopolizing" women from other men who, consequently, have more difficulty attracting a wife or girlfriend. And, women are generally uninterested in having sex with males at the low end of the "mate value" continuum.
As a consequence of greater male reproductive variance, males are playing a riskier, higher stakes reproductive game. They could win big in a way that no woman could ever win, and lose utterly in a way that very few women do. If there is a fundamental, mathematical sex difference in reproductive opportunities and constraints, these differences are likely to lead to different psychological adaptations in the two sexes. If males are playing a higher stakes reproductive game, how has that affected their evolved psychological adaptations? People who play higher stakes games are generally willing to take greater risks.
Is there evidence that men are generally willing to take greater risks than are women? We will explore that in a future post.
For further reading:
Brennan, P. (2010) Sexual Selection. Nature Education Knowledge 1(8), 24
Much Ado About Sex Differences in Reproductive Success, blog post by David Schmitt.
Copyright © Michael Mills
