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It's a scenario that has played out between men and women for centuries.

A man thinks a woman wants sex after she smiles at him. She, meanwhile, interprets his advances as acts of friendship.

Now scientists say they have finally got to the bottom of why this happens - and it's all to do with how male brains have evolved to pick up on the wrong signals.

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Professor Bendixon said that across thousands of generations, women's psychology has evolved to set the bar higher, which means they need much clearer signals than men before they consider sex

A man's ability to reproduce is all about seizing every opportunity. 

He has to spend both money and time on attracting women, which still may not lead to sex.

But it costs even more to not try, because then he won't be able to reproduce at all.

'But that's not how it works for women,' explained researcher Mons Bendixen from the Norwegian University of Science and Technology (NTNU).

A woman can have sex with multiple men over a short period of time without producing any more children.

So for men, it is a low-risk, potentially high-reward situation to have sex with women whenever the opportunity presents itself.

In a recent study, women said that they had acted friendly towards a man and had this misinterpreted as sexual interest about 3.5 times over the past year on average. The familiar scenario has been played out in sitcoms and films, such as this scene between Joey and Rachel in Friends

On the other hand, the cost is potentially great for a woman if she thinks that a man is more sexually interested than she is.

Professor Bendixon said that across thousands of generations, women's psychology has evolved to set the bar higher, which means they need much clearer signals than men before they consider sex.

'Even though these processes aren't conscious, we can still empirically measure the results,' he said.

In a recent study at NTNU, women said that they had acted friendly towards a man and had this misinterpreted as sexual interest about 3.5 times over the past year on average.

The men in the study also reported having been misinterpreted by the opposite sex in this way, but far less often.

'The results are no surprise, seen from an evolutionary perspective,' researcher Mons Bendixen said.

'The fascinating thing is that our results are identical to a study done in the US, even though Norway is one of the most gender-equal, sexually liberal countries in the world.'

'The fact that the hypothesis in evolutionary psychology is supported even when the study is in a society where gender equality is strong, weakens alternative claims that the social roles of men and women in different cultures determine their psychology in these situations,' he added.

Researchers now plan to collect data from high school students to see if the results of this study are also valid for people aged 16-19, and if these differences might lead to sexual harassment.

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