Men may not go on a hormonal rollercoaster with their pregnant partners, but once the baby shows up, the levels of testosterone drop. The hormone drop is due to the property of testosterone, which tends to boost behaviors linked to competing for a mate, risky activities that may conflict with the responsibilities of fatherhood. The biggest testosterone drops were observed in fathers of newborns and those highly invested in child care. The finding that fathers are hardwired to care for children adds to previous cultural models of human evolution, which traditionally depict the mother as being hardwired for hands-on child care.
The study followed 465 men participating in the Cebu Longitudinal Health and Nutrition Survey, started in the Philippines in 1983, when the participants were 1 year old. At age 21.5 (in 2005), the researchers tested the single male participants' testosterone levels when they woke and when they went to sleep. The measurements were repeated at age 26 (in 2009), when about half of the participants had become fathers.
Men who stayed single showed a small age-related decline of about 12 to 15 percent in the male sex hormone, while the testosterone levels of new fathers — those with a baby between 1 month and 1 year — on average dropped about 30 percent. Hormone levels in fathers of newborns (1 month and younger) dropped four to five times lower than levels in single men levels and twice as much as fathers of older children.
Newborn babies come with really intense physical, emotional and psychological changes. This is seen in men's biology responding to that, in line with what is expected in men trying to transition into this new role of being a father to a newborn.
As to the effect of the lowered testosterone, the researchers can't be sure. There could be effects on libido and muscle mass, though they are probably mild, since the participants' levels are still within the normal range.
Lower testosterone could influence the amount of time a man spends with his family, essentially by tempering his urge to go out and reproduce. Higher testosterone has been associated with increased risk-taking and competition with other males. This could be why testosterone levels are even lower with increased child care investment.
The finding may also explain why having a partner and becoming a father are good for a man's health and longevity. This could be somewhat mediated by the changes in testosterone levels. Some researchers believe testosterone lowers immune function: Higher testosterone levels may interfere with the immune system's ability to fight off infection. If this is true, lowering testosterone could be an investment in men's health. The researchers plan on following up with these men at around age 30.
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Do they suggest any of the mechanisms by which this drop could be initiated? e.g. is it a psychological thing, or are there specific cues which cause this? Were there any differences between men who were the biological fathers and those who weren't?
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