Hormone Levels of Married Fathers Become Lower As A Day With Their Families Goes On

Althouse quotes Harvard Magazine:

Analysis showed that marriage, fatherhood, and longer periods spent with wives and children were all linked to lower testosterone levels. Fathers in particular had levels significantly lower than those of unmarried men. Researchers also observed that hormone levels in the morning samples were high and relatively even among the men; the differences appeared at night.

If this study accurately reflects the gender at large, then this is a feature which is designed into men. I wonder: what is the design significance?

Maybe this merely reflects how children will physically tire a person as a day goes on.