The green-eyed monster of jealousy is alive and well - and living in Brazil, according to an international study.
By Dr David Whitehouse
BBC News Online science editor
In relationships, it is well known than men are mostly jealous about sex, while women are mostly concerned about emotional attachments.
Othello, a classic tale of jealousy
Psychologists have conflicting explanations for this, believing it comes either from evolution or from culture. The new cross-cultural research suggests the former is more important.
It reveals that Brazilian men are the most jealous; Swedish men and women are more concerned about sex than any other nation, and Japan is the least jealous country.
Evolution or society
In many cultures, men tend to be most jealous about their partners having sex with someone else. Women, on the other hand, are more likely to be upset about their partners forming an emotional attachment with another party.
It seems that males want to know if their rival was good in bed; females want to know if he loves the "other woman".
Psychologists argue about the source of jealousy. Does it have deep-rooted origins in human evolution, or has it been modified by more recent social changes?
Those in favour of the evolutionary explanation say it could be due to the fact that men can never be absolutely certain that a child is really theirs. Hence, their desire for partners not to have sex with another man.
They also maintain that the origin of jealousy in women is due to the substantial investment they make in time and energy in producing a child. They do not want it to be wasted if their partner falls in love with someone else.
The alternative viewpoint is that men are upset by sex because they think it implies emotional commitment, although they also believe a woman can be emotionally involved without having sex.
Women are upset by emotional infidelity because they believe that for men it automatically means sex, although women believe that men can have sex without commitment.
Evolution and fertility
Gary Brase, working at the University of Sunderland, UK, looked at jealousy in many countries and found the expected differences between men and women.
He found that the biggest difference between men and women was in Brazil; the smallest in Japan. Another finding was that Swedish women were the most concerned about their partners having sex with someone else.
Looking deeper into his survey, Brase noticed that the fertility rate of the country seemed to make a big difference.
Countries with high fertility rates, like Brazil, had men who were very jealous about their partners having sex with others. Men in countries with a lower overall fertility rate, such as Japan, were less bothered.
Brase believes the results support the evolutionary view of the origin of jealous behaviour.