See https://www.amazon.com/Price-Motherhood-Important-World-Valu... for documentation of this fact.
(Speaking anecdotally, I came out much worse off for my first marriage and that was mostly my fault. My second marriage has been a delight though.)
That said anyone quoting statistics should be able to back them up. I encountered the custody figure, along with a lot of other useful information about how the results of divorce are harder on women than men, in http://www.amazon.com/Price-Motherhood-Important-World-Value.... It is an interesting read, though not exactly the sort of thing you want your wife to encounter during her first pregnancy. (Which is how I learned about that book...)
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