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Testosterone and Fatherhood

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Men’s testosterone levels drop significantly when they become fathers.


Transcript

Dialing back Dad’s testosterone.  I’m Bob Hirshon and this is Science Update.

When a man becomes a dad, his future includes more responsibility—and less testosterone. This according to Northwestern University anthropologist Lee Gettler and his colleagues. Past studies had found that fathers tend to have lower testosterone levels than childless men.

Gettler:
The question remained whether men with low testosterone were simply more likely to become fathers, or whether fatherhood sort of caused a decline in testosterone, once men became parents.

Gettler’s team followed more than 600 Filipino men from age 21 to 26.  All of them started out single. But those who ended up becoming fathers experienced greater drops in testosterone during that time than those who didn’t. The drops were steeper still among dads who helped with child care. Gettler says the findings are consistent with animal data, and the idea that high testosterone may promote behavior that interferes with parenting. I’m Bob Hirshon for AAAS, the Science Society.


Making Sense of the Research

This study illustrates how scientists determine cause and effect. Correlation, or the tendency of two different things to occur together, is different from causation, or the tendency of one thing to result in another. For example, if it's raining out, the ground will almost certainly be wet. That's a causal relationship, because the rain makes the ground wet. But if the ground is wet, you will also see many more people with umbrellas outside than you would if the ground were dry. That's a correlation—umbrellas tend to occur along with wet ground—but there's no causal relationship. The umbrellas don't make the ground wet, or vice versa. They just go together.

Previously, scientists had found that when they compared men of similar ages in a given population, fathers tended to have lower testosterone levels than non-fathers. That could mean that becoming a father lowers your testosterone. But the data didn't prove or disprove that causal relationship. As you heard, it might have simply been that men who have lower testosterone levels are more likely to become fathers than men with higher testosterone.

Gettler and his colleagues used a prospective study to try and settle the question of causality. Rather than simply compare people to each other at one particular time, a prospective study follows the same people for months or years. That way, you can compare the participants not only to one another, but also to earlier versions of themselves. Prospective studies are better at determining causality than single-occasion studies, because they can measure how people change after being exposed to whatever it is you're studying. The downside of prospective studies is that they're more difficult and costly than single-occasion studies.

Gettler's team started with a group of men who were all 21 years old and single. Of course, some of those men had higher testosterone levels than others from the beginning. And across the board, the same men had somewhat lower testosterone levels by the time they reached 26. 

The study focused on the relationship between fatherhood and the size of that drop in testosterone. The researchers found that testosterone declined more in men who became fathers during the study period than among men who didn't. Within the group of fathers, men who provided signifiant child care had even greater decreases in testosterone than those who didn't help out much.

The results support the idea that fatherhood directly causes testosterone to drop. How or why that happens remains to be seen. But similar trends have been seen in animals, and behavior theorists believe that the behaviors testosterone promotes, including promiscuity, competitiveness, and aggression, may be useful for finding a mate but counterproductive for raising a family. That's consistent with another piece of evidence from the human study: that men who started off with relatively high testosterone levels were more likely to become fathers during the study period than those who started lower.

Now try and answer these questions:

  1. What is a prospective study? How does it differ from a simple comparison?
  2. What was the correlation this study looked into? How did it use a prospective sample to investigate causation?
  3. Assuming the study findings are accurate, are there explanations for the result besides the idea that fatherhood causes testosterone to drop?
  4. Can you give other examples of causal and non-causal correlations?

You may want to check out the September 30, 2011, Science Update Podcast to hear further information about this Science Update and the other programs for that week. This podcast's topics include: HORMONES & BEHAVIOR – Testosterone and fatherhood, the genetics of oxytocin and depression, gender and spatial reasoning revisited, where taste is found in the brain, and more.

The Science Update Asymmetry and Aggression also delves into the difference between correlation and causation.

The Science Update Testosterone Tradeoff focuses on an experiment that tested the effects of testosterone on the mating habits of birds called juncos.


Going Further


For Educators

The Science Update lesson Asymmetry and Aggression also delves into the difference between correlation and causation.

The Science Update lesson Testosterone Tradeoff focuses on an experiment that tested the effects of testosterone on the mating habits of birds called juncos.


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