24.8: Hormones can affect behavior.

It can be hard to imagine how a little more or a little less of a chemical could influence an animal’s behavior, but experimental research on hormones and behavior has documented literally thousands of such effects. The scholarly journal Hormones and Behavior, for example, in its 67th volume as of 2015, publishes five issues a year, each containing 25 or more articles describing laboratory and field studies on hormones and their influences on the development and expression of behaviors (FIGURE 24-20). In these studies, researchers take a variety of experimental approaches. Just two of these are described here.

Genetic manipulation of hormone levels. Researchers bred some lab mice that were unable to make the enzyme aromatase, which is essential to the production of estrogen. The mice with this deficiency had two striking behavioral differences from typical mice: they ran excessively on their exercise wheels and, when sprayed lightly with water, spent significantly longer grooming themselves. Both of these behaviors are indicators of obsessive-compulsive disorder in mice.

Physiological supplementation of hormone levels. Researchers implanted testosterone capsules in male juncos, a type of songbird. These males produced a song that was more attractive to females, and they produced more offspring than control-group males that received a capsule containing no testosterone. (In similar studies, male birds with supplemented testosterone tended to have increased muscle mass and were able to establish and maintain larger territories than non-supplemented males.) However, male birds with testosterone implants spent less time with their offspring and gave the offspring less food. The testosterone-supplemented birds were also more susceptible to disease and had shorter life spans.

The dramatic and close link between hormones and behavior has caused people to debate whether sex offenders should be castrated to modify their behavior (FIGURE 24-21). Whether or not castration—removal of the testes, with a resultant near-complete drop in testosterone levels—can reliably rehabilitate violent sex offenders is intensely debated. In the Czech Republic, one of the very few countries where sex offenders may be castrated, 94 prisoners underwent castration between 1998 and 2008. None of these men have been reported to commit any further offenses. Similarly, in a Danish study of 900 castrated sex offenders, the rate of repeat offense dropped to approximately 2%, from close to 80% among non-castrated sex offenders.

Opponents of the castration of sex offenders argue that the evidence cannot be trusted, because it relies partly on self-reporting by the castrated men. Moreover, the opponents of castration argue that having such an option is coercive, because many convicted sex offenders will feel obligated to opt for the surgery, leading to a violation of the prisoners’ rights. Some argue for the reversible form of castration by injection of chemicals that block the effects of testosterone, but this is opposed by many on the grounds that it relies on the released offenders voluntarily undergoing treatment, which they may stop at any point.