Another way, practiced commonly among slave-owning dynasties in the American South, was to keep marriage within the family. Nancy Wilmsen Thornhill of the University of New Mexico has shown how in such families more often than not men married their first cousins. By tracing the genealogies of four southern families, she found that fully half of all marriages involved kin or sister exchange (two brothers marrying two sisters). By contrast, in northern families at the same time, only 6 percent of marriages involved kin. What makes this result especially intriguing is that Thornhill had predicted
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