By Satoshi Kanazawa in The Scientific Fundamentalist:

... What is interesting about McNulty et al.’s study is that it shows
that the absolute levels of physical attractiveness of the husband and
the wife are not as important as their relative difference (whether the
husband or the wife is more attractive). In fact, in their data, once
the relative difference is taken into consideration, the absolute
levels make no difference for the couple’s marital satisfaction
or their behavior. Their conclusion is that couples in which the woman
is more attractive than the man are happier than the couples in which
the man is more attractive than the woman. Why is this?

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