Most women have probably experienced a time (or a handful of times) when her politeness — a simple chuckle at a joke, a comforting pat on the shoulder, or even just showing the slightest interest in a conversation — has been wrongly interpreted as flirtatiousness.
Straight men, research has shown, are a lot more likely than straight women to convince themselves that someone is romantically interested in them when they, in fact, aren't.
But why does this phenomenon occur? Well, there are two theories, and of course, it's a classic battle between nature and nurture.

Error management theory says that men have been biologically wired to "overperceive" sexual interest from women so that they never miss out on an opportunity to plant their seed. Women, on the other hand, have evolved to "underperceive" sexual interest so they don't risk being impregnated by a flaky guy.
On the other side of the argument is the social-roles theory, which says that societal norms determine how men and women interact. So misconceptions are likely to be higher among men in places that lack gender equality, while the levels of misinterpretation between men and women are probably more similar in places with better gender equality. 

In order to get to the bottom of this mystery, Mons Bendixen, a psychologist at the Norwegian University of Science and Technology, decided to replicate a 2003 U.S. study but this time in Norway, which has a society that is generally considered to be more egalitarian.

"Norway is very sexually liberal compared to the USA. A Norwegian woman can play a more active and proactive part in the dating game than an American one without being subject to the same degree of derogation," Bendixen explained to New York Magazine.

To test out this hypothesis, Bendixen recruited 308 heterosexual university students between the ages of 18 and 30 and asked them all the same set of questions. He found that 88% of women reported their friendliness had been misinterpreted as some sort of come-on, while only 70.6% of men reported the same. These results are very similar to those of the U.S.-based study. 

According to Bendixen, this means that the error management theory (nature) wins over the social-roles theory (nurture). In other words, men's interpretation of flirtatious signals has more to do with evolution than unequal opportunities for men and women or a misogynistic culture. 

So whether or not this information helps you handle the smile that creeps onto a dude's face when you're just trying to be nice to him, that's up to you. As you roll your eyes for the umpteenth time, try to keep in mind that he's still got a little caveman in him.