What is the implied model behind assortative mating?

In a fascinating new paper by Brant, et.al. on IQ, I read the following bit:

…we found that higher-IQ parents actually showed less assortative mating: the difference between parental IQ scores was positively correlated with mean parental IQ score.

My questions are numerous but I will start with two.

First, is this the correct metric for “less” assortative mating?

Second, do “straightforward” models predict such a result as a matter of course?  For instance, higher-IQ individuals may have greater scope to choose mates on the basis of complementary skills.  That may imply higher IQ gaps.  Furthermore higher-IQ individuals may marry later in life, put more effort into choosing, and encounter a wider variety of potential partners.  That may also imply wider gaps in IQ across partners, even if assortative mating (as defined in an all things considered way) remains strong.

Very often there is more variability at the tops of distributions rather than at the bottoms or in the middles.  Yet “pull away” forces may continue to operate.

To present a simple analogy, income gaps between marrying couples are probably higher at the upper end of the distribution than at the lower end.  Yet assortative mating with respect to income still may reenforce income inequality.


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