In the original 2003 research on the topic, economists Gordon Dahl, from the University of California-San Diego, and Enrico Moretti, at UC Berkeley, found that couples with a first-born girl were about 5 percent more likely to divorce than parents of a first-born boy. When there are as many as three daughters that difference spiked to 10 percent.
Researchers don't know the cause, but have proposed many possibilities, from the presence of daughters making a mother more likely to leave an abusive husband to the idea that a man is more likely to marry a woman who is pregnant with his son. In any case, the statistics do not necessarily reflect the odds for an individual marriage. Link
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