Science Watch

Which would upset you more: (A) Finding out that your romantic partner was having passionate sex with someone else, or (B) Finding out that your partner was falling in love with someone else?

If you answered, "both," you are not alone; many people find it difficult to disentangle sexual and emotional infidelity. But when people are forced to choose, clear sex differences emerge. Studies consistently find that, across cultures, age groups and populations with varying amounts of sexual experience, women are more likely than men to choose "B," the option corresponding to emotional infidelity.

Psychologists disagree over the cause of the difference. Some, like David Buss, PhD, a professor of psychology at the University of Texas at Austin, and Todd Shackelford, PhD, of Florida Atlantic University, believe that jealousy is an evolutionary adaptation to the different reproductive challenges faced by men and women. According to this theory, men are most concerned about sexual infidelity because of the risk of inadvertently raising another man's child, while women are most concerned about emotional infidelity because of the threat of losing their mate's support. Both sexes, the theory suggests, should have evolved special sensitivities to the type of infidelity--sexual or emotional--that threatens them most.

The theory has been widely accepted since Buss first provided evidence for it in 1992, but some psychologists continue to question it. Northeastern University psychologist David DeSteno, PhD, and University of California, San Diego, psychologist Christine Harris, PhD, among others, have argued that the evidentiary basis of the theory is weak.

Now, DeSteno and Yale University psychologist Peter Salovey, PhD, have suggested that some of the strongest evidence in favor of the theory--answers to forced-choice questions like the one at the beginning of this article--may be a methodological artifact. The study was co-authored by Northeastern University graduate students Monica Bartlett and Julia Braverman and will appear in the November issue of the Journal of Personality and Social Psychology (Vol. 83, No. 4).

"What we found is that the same people on a forced-choice questionnaire will show a sex difference, and on every other type of questionnaire that we give them will not show a sex difference," says DeSteno. He and Salovey also found that the sex differences disappeared when participants had to perform a second task simultaneously. "What you find is that, under cognitive load, women reverse," says DeSteno. "All of a sudden, women, like men, are saying, 'Sexual infidelity bothers me more than emotional infidelity.' And that brings women's response into line with all the other measures."

Buss and Shackelford are skeptical of the study. They see it as a flawed contribution to a debate that, they believe, still leans in favor of the evolutionary account. "The way to evaluate these things is always to look at the weight of the scientific evidence," says Buss. "I think what they do in this paper is they take a very, very narrow view and ignore the bulk of the scientific evidence that's out there."

Method matters

DeSteno believes that the evolutionary forces that shaped jealousy are unlikely to have differed between men and women. "If there is a biological trigger, or part of jealousy shaped by evolution, it's probably not going to operate differently among the sexes, because the benefits from both romantic and nonromantic relationships are important to men and women at all stages of their lives," he says.

In 1996, DeSteno and Salovey suggested that gender differences in jealousy exist not because of specific, evolved mechanisms, but because the sexes have different expectations about the implications of sexual and emotional infidelity. According to this "double-shot" hypothesis, women believe that men can have sex without emotional commitment, but not emotional commitment without sex, while men are evenly split on whether the same applies to women. Those expectations, the theory suggests, explain why an individual of either sex would find one type of infidelity more upsetting than the other.

The idea was supported in studies by DeSteno and Salovey and, separately, by Harris. But in 1999, Buss and his colleagues showed that sex differences in jealousy persisted even when the question was rephrased to eliminate the possibility of one kind of infidelity leading to the other. They also pointed out that the evolution could have shaped gender differences indirectly by instilling in men and women what DeSteno and Salovey had called "differential infidelity implications." The findings left the double-shot hypothesis in limbo and motivated DeSteno and Salovey to look for other weaknesses in the evolutionary account.

"We decided that the one way that we could really put this debate to some level of conclusion is not to suggest alternative mechanisms," says DeSteno, "[but instead to] put the evolutionary hypothesis to a very stringent test, and test it on its own merits." Previous studies had suggested that forced-choice questions were the only ones on which sex differences were found consistently. DeSteno suspected that the forced-choice format encouraged people to consider carefully the trade-offs between each type of infidelity--a thought process unlikely to take place in real-world situations.

To test the hypothesis, he and Salovey conducted an experiment in which they asked participants about their responses to infidelity in a variety of ways, including forced-choice questions, continuous-scale measures and checklists of emotions. The experiment's results confirmed their suspicions: The only measure that showed the predicted sex difference was the forced-choice question, on which 66 percent of women but only 46 percent of men chose emotional infidelity as the most upsetting option. None of the other measures showed the same interaction between sex and type of infidelity. On continuous-scale measures, both men and women rated sexual infidelity as most upsetting. A conceptual replication on the Web site found similar results in a sample of 22,000 people.

They then conducted a second experiment to explain why the forced-choice question gave results that were so different from those of the other measures. DeSteno and Salovey hypothesized that if a sex difference in jealousy is caused by a specific evolved mechanism, it should be automatic. Interfering with higher thought processes by imposing a cognitive load on participants should have no effect on the strength of the difference; possibly, it could even increase it.

In the experiment's no-load condition, which resembled the forced-choice component of the first experiment, the researchers found the expected sex difference: Women were more likely than men to select emotional infidelity as the most upsetting option. In the load condition, however, the women's behavior changed: Instead of choosing sexual infidelity only 36 percent of the time, as they had in the no-load condition, they now selected it 65 percent of the time. In both conditions, almost all of the men picked sexual infidelity as the most upsetting option.

If the results had been random--if participants had chosen sexual and emotional infidelity equally often--then the experiment would have demonstrated little, since it could have been argued that cognitive load was preventing participants from even considering the question. Instead, the tendency of both sexes to choose sexual infidelity showed that that participants were still answering the question, but that different psychological mechanisms were now coming into play.

Buss and DeSteno can each point to research that supports his position and undermines the other's, so whether or not the study will have a major impact on the status of the evolutionary account remains to be seen.

Human nature

One reason the debate about jealousy has been so heated is that it raises questions about the fundamental causes of human behavior.

"At the core is the question about the mutability of human nature," says W. Gerrod Parrott, PhD, a professor of psychology at Georgetown University. "Is there something that's so strongly built into human nature--like a gender difference in what makes you feel jealousy in romantic relationships--that's going to turn up despite all the differences in the experience people have as they grow up, and all the differences among world cultures?"

Buss and Shackelford believe that critics of evolutionary psychology are wedded to a long-discredited "blank slate" view of human nature, one that denies the importance of biology and evolution. "It's a fundamental difference in conceptions of human nature," says Buss. "I think the cumulative weight of the evidence is showing that that view is crumbling, and some people just don't want to let it go."

Salovey disputes the idea that he or DeSteno are against evolutionary explanations of human behavior. "I sometimes think of it as a false dichotomy," he says. "Much of our behavior may be the expression of an evolutionary tendency at its core, [but] I put a lot of faith in our reasoning abilities and in social context in modifying, shaping--sometimes even stamping out or exacerbating--these evolutionary tendencies."

DeSteno agrees. "I think the evolutionary perspective, in general, has a lot to offer," he says. "However, in the case of jealousy, I think the longstanding view is in error."