In my back-and-forth with Nicole Lapin, we rediscovered a widely reported fact: women apparently value higher IQs in men more than men do in women.

That is to say, men seem to be more willing to date and marry those to the left of them on the intelligence curve than women are. As a result, some highly intelligent women find themselves left out in the cold because some of the men they would like to date and marry are off gallivanting with a less intelligent woman.

So why do women place a higher value on a mates intelligence than men do?

The answer is one word: fidelity.

As it turns out, intelligence in men is correlated with valuing sexual exclusivity. A 2010 study by London School of Economics professor Santoshi Kanazawa established the relationship. More intelligent American boys are more likely to grow up to value sexual exclusivity in early adulthood than less intelligent boys.

So the unwillingness of women to date less intelligent men is rational if they are seeking a mate who values monogamy and sexual exclusivity.

But if intelligence and sexual exclusivity go hand in hand, why are men willing to date less intelligent women? Well, it turns out Santoshi's study proved something very interesting: for women, there is no statistically significant correlation between intelligence and valuing monogamy. Smarter women are no more likely to value sexual exclusivity than less intelligent women. Or, to flip it around, less intelligent women do not place a lower value on monogamy than more intelligent women.

In other words, when women date less intelligent men they pay a cost: a diminished interest in sexual fidelity. Men don't pay this cost, making dating across the intelligence curve easier.

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