A SINGLE act of rape may be more than twice as likely to make a woman pregnant as a single act of consensual sex.
That statistic will reopen the hotly contested debate over whether rape can be a successful reproductive strategy in evolutionary terms. It could help to explain why men raping women has been so common throughout history and across cultures, two American researchers told the conference. Previous studies found that rates of pregnancy resulting from rape could be anything up to 30 per cent, compared to a 2 to 4 per cent chance of getting pregnant from a single act of consensual sex. This led some biologists, notably Randy Thornhill from the University of New Mexico at Albuquerque, to parade the figures as evidence that rape is a natural way for men to spread their genes (New Scientist, 19 February 2000, p 44).
But in the ensuing controversy, the studies were all roundly criticised. Some rely on crime statistics, which may skew the figures: rape victims who become pregnant may be more likely to report the crime than those who do not. Others fail to measure the influence of contraception-either preventive or post-coital, such as the morning after pill. And some include acts of oral and anal rape, which cannot result in pregnancy.
Jon Gottschall, a researcher at St Lawrence University in Canton, New York, says the studies failed to answer the crucial question: "What is the evolutionary success of rapists?"
To find out, he and his wife Tiffany Gottschall examined the results of National Violence against Women Survey, a study by the National Institute of Justice and the Centers for Disease Control and Prevention. The women studied were phoned at random and interviewed about their experiences. The Gottschalls focused on 405 women who had suffered a single incidence of penile-vaginal rape at some point between the ages of 12 and 45. Of these, 6.4 per cent became pregnant. But that figure jumped to nearly 8 per cent when the researchers allowed for the women who'd been using birth control-US government statistics show that 1 in 5 of the women in the sample were likely to have been using the pill or an IUD.
To complete the comparison, the Gottschalls needed to know how many women in that age group get pregnant from one-night stands and other one-off acts of consensual sex. The answer-reported this year in a separate study by Allen Wilcox, head of the epidemiology branch of the National Institute of Environmental Health Sciences-was a mere 3.1 per cent. "It was surprising to see this margin of difference," says Jon Gottschall.
Crucially, he believes the difference cannot be explained away by the argument that women having regular consensual sex are, on average, more likely to be using contraception than rape victims. "All the women in Wilcox's study were trying to get pregnant, and not taking precautions," he says. A more likely interpretation, say the Gottschalls, is that rape really does result in more pregnancies.
One possibility is that women feel more attractive and sexy when ovulating, and unconsciously give off signals that rapists might pick up-although it's unclear whether men do in fact notice these signals (see p 12). Another, more likely explanation is that rapists target attractive and healthy-looking women-both characteristics that can indicate fertility. But whatever the reason, say the researchers, none of this absolves the rapist or means the victim is in some way to blame. However, sociologist Frank Furedi of the University of Kent at Canterbury says that trying to answer questions about the reproductive success of rape is essentially meaningless. He believes that what constitutes rape or a consensual sexual act within a relationship can be extremely ambiguous and says that phone surveys often push women into claiming they have been raped. "When relationships sour, women often redefine them in a destructive form," adds Furedi. Rape has also meant different things through history. "Considering it as a strategy is essentially nonsense." Gottschall says women may also sometimes reinterpret a rape as a consensual act years later when the relationship has improved. He agrees that the data in the surveys is not perfect, "but the best information we have still all points in one direction".
One other question remains unanswered. For this form of rape to be a successful evolutionary strategy, the benefits of the crime have to outweigh the costs for the rapist. And the costs are extremely hard to judge. However, some surveys suggest that less than 1 per cent of rapists are convicted in the US, says Jon Gottschall. Even in traditional societies, a high proportion of rapists may have never been punished because of the costs to the victim of reporting the crime.
Author, Matt Walker
New Scientist reports from the Human Behavior and Evolution Conference in London
New Scientist issue: 23rd June 2001
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