In the first study of its kind, researchers compared the testosterone level of men when they were single and childless to their testosterone level after they had had a child.

While testosterone always decreases when men age, the decline was much more pronounced among the men who had become fathers.

In fact, the more a father was engaged in child-rearing activities, such as changing diapers, the more his testosterone level dropped.

Scientists believe this decline is an evolutionary response to men needing higher levels of testosterone when they are single and out there wooing women, but lower levels when they are dealing with the more nurturing demands of child care and sustaining a family life.

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