If your partner has sex with someone else, it is considered infidelity - even if no emotions are involved. But it is also considered infidelity when your significant other develops a close personal relationship with someone else, even if there is no sex or physical intimacy involved.

A recent Norwegian study shows that men and women react differently to various types of infidelity. Whereas men are most jealous of sexual infidelity, so-called emotional infidelity is what makes women the most jealous. Evolutionary psychology may help explain why this may be.

Significant gender differences

"Men and women's psychology is similar in most areas - but not when it comes to reproduction," says Associate Professor Mons Bendixen, from the Norwegian University of Science and Technology's (NTNU) Department of Psychology. He has teamed up with NTNU Professor Leif Edward Ottesen Kennair and Professor David Buss at the University of Texas, Austin to publish their study of jealousy involving more than 1,000 participants.

Although the evolutionary psychologists had expected women and men to respond differently to questions about infidelity and jealousy, they were surprised that the differences were so strong.

Norway is one of the world's countries with the greatest degree of gender equality. Fathers are expected to be there for their children, from changing diapers to childcare. Norwegian paternity leave and other legal regulations send the message that men should invest time in their families. At the same time support for single parents makes it possible to raise children alone if dads don't pull their weight.

And yet, even in Norway's culture of substantial gender equality, large sex differences persist in what triggers jealousy in men and women.

Is he the child's father?

Recent research on jealousy considers two main types of infidelity: Having sex with a person outside the relationship, or developing an emotional attachment to a person outside the relationship

Psychology has two contrasting theoretical perspectives on men and women's emotional responses to infidelity. The first has its roots in cultural gender roles while the other takes an evolutionary psychology perspective.

The first perspective maintains that in a culture with a high degree of equality, men and women interpret the world similarly because of more equality in socialization and gender roles, than is the case in a culture with a low degree of equality. According to this approach, the human mind is largely shaped by the different roles that cultures assign to women and men and the experiences they have in those roles.

The evolutionary perspective is different. According to this approach, men and women over thousands of generations have had to adapt to different challenges that are related to reproduction. Infidelity is one such challenge. A man must decide whether he really is the father of his partner's child, and if he should choose to invest all his protection and status resources on this child. Since the dawn of time men have grappled with paternity insecurity, since fertilization occurs inside a woman's body.

According to the evolutionary psychology explanation, men's jealousy is an emotional reaction to signs of sexual infidelity. The jealousy serves to reduce the chances that his partner is cheating, since he then monitors her more closely.

Our ancestral mothers chose carefully

It's a different story for the child's mother. She knows for sure that she is the child's mother, but she must ensure that the child's father will provide their offspring with food and the security and social status it needs. The greatest threat for the woman is not that the man has sex with other women, but that he spends time and resources on women other than her.

Women today are descendants of women who over thousands of generations have reacted with jealousy to men who sent signals that they were less invested in them. Evolutionary psychologists believe that women are especially sensitive to signs that the man is devoting time and attention to other women.

According to Bendixen, women who were indifferent to whether a man was emotionally attached to other women were more likely to have to take care of the child without his resources. Men who were indifferent to whether the woman had sex with others and who therefore invested resources on other men's children, ended up passing on fewer of their genes. We are descendants of men and women who have responded appropriately to these threats, says Bendixen.

He adds that neither past experiences with infidelity nor whether we are in a relationship seem to affect men's and women's reactions to infidelity.

"The cultural gender role perspective believes that jealousy is learned, but we feel confident that these reactions are mechanisms that are part of an evolved human mind, given comparable findings across several nations," Bendixen says.

Two different measurement methods

In the recent study, published in the November 2015 issue of Personality and Individual Differences, participants were randomly given one of four versions of a questionnaire about jealousy. Half the respondents were asked to check off whether the emotional or sexual aspect of infidelity was the most upsetting to them in four different infidelity scenarios - a so-called "forced choice" paradigm.

The other half rated the scenarios using a continuous measure; they were asked to report on a scale from 1 (not at all) to 7 (very) how jealous or upset they were when the scenarios described either emotional or sexual infidelity.

In addition, the order of the questions was changed in half of the forms, so some respondents were asked about their experiences with infidelity before they answered the scenario questions. The remaining respondents answered these questions after the scenario questions. This manipulation turned out to have no effect on how participants responded.

"As in two of our previous studies, we found clear sex differences in the jealousy responses among those who had to choose which aspect of infidelity was most upsetting to them. We also found similar sex differences when we used a continuous measure paradigm. These sex differences are remarkable, since they were obtained using two alternative methods of measurement, and in a highly egalitarian nation with high paternal investment expectancy," Bendixen said.