Men appear to be biologically wired to care for their babies, say researchers who have discovered levels of testosterone go down after fatherhood.
This drop in the male hormone presumably makes the dad more family-oriented and less likely to stray, say the Northwestern University team.
Testosterone increases a man's sex drive and helps him compete for a mate.
The Proceedings of the National Academy of Sciences work followed 624 young men before and after they became fathers.
This revealed that as soon as a man had a baby, his testosterone levels dropped substantially.
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