WORLD SCIENCE

"Long before it's in the papers"
December 19, 2005

Oct. 31, 2005
Special to World Science

A British scientist has stirred controversy with a new study that finds battered women have more sons than average women do. 

The women, he suggests, are unwitting pawns of a perverse evolutionary mechanism that polluted the population with more wife-beaters among our ancestors. 

The fact that violent men and wife-beaters today often go to jail, Kanazawa argues, has helped interrupt the evolutionary mechanism that keeps spreading their genes.

But some researchers doubt the conclusion.

The study’s key finding was that “violent men have more sons,” wrote the author of a paper detailing the results, Satoshi Kanazawa of the London School of Economics and Political Science. The paper appeared in the Oct. 20 advance online issue of the Journal of Theoretical Biology.

The phenomenon has perpetuated violence and wife-beating in the population, he added, given that violent tendencies are known to have a strong genetic component.

Kanazawa argues that history bears out the claim. “The most prolific father in recorded history, with at least 1,042 children, was the emperor of Morocco in the late 16th and early 17th century named Moulay Ismail the Bloodthirsty,” wrote Kanazawa, who added the italics himself. The monarch “was reputed to have killed 30,000 people by his own hands.”

In ancient and prehistoric times, violence often paid off through increased mating success, Kanazawa argues. 

Today, violent men seldom achieve such success, and often meet with jail sentences instead, he notes; yet echoes of the evolutionary mechanism remain. His study found that wife-beaters today have significantly more sons than daughters. Although parents everywhere have slightly more sons than daughters on average, he found the proportion is even higher among wife-beaters.

Kanazawa said this backs up a hypothesis he has developed, which holds that parents have an innate ability to manipulate the sex of their children somewhat before their birth. Unconsciously, parents will skew the normal boy-to-girl ratio in favor of one that maximizes their children’s reproductive success, and thus best helps spread their genes.

In times when violence pays off, women mated to violent men will have more sons than daughters, Kanazawa’s hypothesis holds. This is because violent sons are good reproducers, whereas violent women aren’t. “Men tend not to be attracted to violent women, and violent women tend not to make good mothers,” Kanazawa wrote in an email.

If the women were mated to peaceable men, Kanazawa adds, they would instead produce more daughters than sons, because women have greater success in attracting mates when they are good empathizers. “Parents unconsciously ‘bias’ their sex ratio toward sons if they are violent, and toward daughters if they are not violent,” Kanazawa said in the email.

The phenomenon probably no longer works to spread any “violence genes,” Kanazawa noted. “These violent men are still having more sons than daughters. However, their sons will not do well in our current societies, if they are also violent like their fathers, and therefore will not attract mates.”

Kanazawa drew his findings from an analysis of various public health studies covering thousands of American and British women and their children. These included a group of 3,528 randomly picked children of battered women in the United States, and a second group of 862 battered women, and their children, from the United Kingdom.

Because the information available was different for these groups, Kanazawa said he analyzed the data in different ways. But the result always pointed the same way. 

The American children of battered women contained a percentage of males more than 3 percent higher than normal, Kanazawa found. The British women spawned males at a 7 percent higher rate than non-battered women, whereas they gave birth to females at a lower rate, slightly below 1 percent.

But other researchers say there are simpler ways to account for these findings than through Kanazawa’s explanation.

“The findings he reports are what you would otherwise predict even if his theory was false,” said Jeremy Freese, an assistant professor of sociology at the University of Wisconsin-Madison, in an email. 

This, he explained, is because the gender of children can influence the parents’ relationship as much as or more than the other way around. This can lead in various ways to the results Kanazawa found. 

For instance, “There is good reason to think that couples are more likely to stay married when they have sons versus when they have daughters,” Freese continued. Thus “couples with daughters could be more likely to divorce before a bad relationship becomes physically abusive than couples with sons.”

Kanazawa’s theory is an elaboration of a hypothesis first proposed in 1973, which has also failed to find much support from subsequent research, Freese added. Kanazawa maintains that some studies have backed up that hypothesis, called the Trivers-Willard Hypothesis, depending on whether they use the right methodology.

The original hypothesis held that parents can manipulate the sex of the children depending on how well-off they are, because depending on the circumstances, either more sons or more daughters can be the best way to spread the parents’ genes. Kanazawa has proposed that a range of other situations besides material success can skew children’s sex ratio in a family.

Kanazawa “is one of the most creative and interesting sociological theorists I know,” Freese said. “Creative theorists who generate a lot of novel and interesting ideas typically end up having many of those ideas turn out to be wrong, which is perfectly fine because of the innovative contributions they make when they’re right. This time, my bet is that Kanazawa’s wrong.”

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