Adult heterosexual women and men are often jealous about completely different threats to their relationship. These differences seem to establish themselves far sooner than people need them. The finding surprised researchers at the Norwegian University of Science and Technology (NTNU) who studied the topic.

"You don't really need this jealousy until you need to protect yourself from being deceived," says Professor Leif Edward Ottesen Kennair at NTNU's Department of Psychology.

Romantic jealousy can be experienced as horrible at its worst. But jealousy associated with a partner's infidelity has clearly been an evolutionary advantage.

Jealousy is activated when a relationship we care about is threatened. The function is probably to minimize threats to this relationship. These threats have historically been somewhat different for men and women."

Per Helge H. Larsen, Master's Student, Department of Psychology, Norwegian University of Science and Technology

Evolutionary psychology can help explain the gender differences having to do with this jealousy.

The differences in sexual jealousy between the sexes, simply put, revolve around the possibilities for their own children. Previous research has already established that:

Men more often react more negatively when their partner has had sex with others than if she falls in love or spends time with someone without having sex.

It's easy to explain: if the woman is sexually unfaithful, it ultimately means that her partner might need to use his own resources to raise another man's children.

Women, on the other hand, are always sure that the child is theirs. They tend to react more negatively to their partner having feelings for another woman than that he's had sex with her.

This response can also be explained. Historically, she could suffer a loss of resources and status for herself and their child if he left her for someone else.

We should note that these differences have been with us since long before birth control pills and the possibility for women to feed and raise their children alone. A few generations aren't enough to change either biology or culture very much.

The gender differences that lead to jealousy are easy to explain. They are evolutionary adaptations that get passed on to the next generation – but why does this gender difference arise so early?

Precisely this question presents theoretical challenges for the researchers, because jealousy has historically not been risk-free, either.

"Jealousy is potentially a costly reaction, perhaps especially for the man before he is physically strong enough to defend himself and his partner against rivals, and before he would normally have had the opportunity to have a steady partner through marriage," says Kennair.

Throughout history, jealous boys and men have run a great risk by expressing their jealousy. Being ostracized, injured or killed in competing for women is all too well known.

"Throughout evolutionary history, the usefulness of man's form of jealousy would probably have been reserved for men of high status who had a great ability to defend themselves," says Kennair.

So why be jealous before you're able to take care of your partner?

"We knew that this difference becomes established in the early 20s, but through our study we've shown that it appears even earlier," says Larsen.

The research group at NTNU wanted to find out when these gender differences around jealousy, sex and emotions begin.

To this end they studied 1266 pupils aged 16 to 19 years in upper secondary school. However, it turns out the participants weren't young enough for the researchers to answer this question as to when gender differences develop.

"The gender difference was stable and clear throughout the age range of the study. This is pretty startling," says Professor Mons Bendixen in the Department of Psychology.

"The gender difference wasn't affected by whether the teens currently had a boyfriend or girlfriend, or whether they had made their sexual debut. The difference thus doesn't seem to have anything to do with experience," Bendixen adds.

We can imagine, and perhaps assume, that the gender differences in jealousy responses arise even earlier than age 16. But we don't know that for sure yet. To confirm it, we need to study even younger boys and girls.

"It's also unclear how young study participants can be to research this in a meaningful way," says Kennair.

Distinguishing between sexual jealousy and other types of jealousy can quickly become meaningless for the very youngest among us.

In one way or another, the benefits of this early, gender-specific sexual jealousy must have outweighed its dangers.

"It could be that the early development of sexual jealousy is simply preparing us for adulthood, and that it has no other function at a younger age."

But Kennair emphasized that jealousy is a dangerous feeling. Young men could put themselves in danger by experiencing this feeling before it was appropriate and they were physically strong enough to defend the relationship.

But the researchers are clear that this idea is still speculation.

"We need further research and theory development on the basis of these findings," Kennair said.