A recent study conducted in the U.K. reported that professional male rugby players who watched a brief erotic video (a clip featuring exotic dancing) before going to the gym showed increased performance on a squat workout. A similar effect was produced by watching a video clip of big rugby hits. Watching a sad video clip showing starving children in Africa, instead, was associated with low performance on the workout.

What is the explanation for these results? It turns out that watching the erotic or the aggressive video clip increased the rugby players' levels of testosterone, while watching the sad video reduced their testosterone. Testosterone is known to increase muscle mass, physical strength, and athletic performance. Testosterone can also increase the motivation to compete or to fight. In this case, the researchers who conducted the study suggested that the men whose testosterone increased after watching erotic or aggressive video clips probably felt stronger and tried to lift a heavier weight during the workout (a 3 repetition maximum lift on a free-weight back squat).

The study was conducted with 12 male rugby players in their 20s, and took place a few minutes before they headed to the gym. The men were tested repeatedly over the course of several days. Each man was asked to watch a different 4-minute video clip on a computer on different days. As a control condition, the men stared at a blank computer screen for the same amount of time. Their testosterone levels were measured in saliva samples collected right before and 10 minutes after the videos. The quick increase in testosterone that occurred after watching the erotic or the aggressive videos was accompanied by lifting weights that were, on average, 5-10 kg heavier than those lifted after watching the sad video or staring at the blank computer screen.

Previous studies had shown that testosterone levels increase in both men and women during the minutes or hours preceding the beginning of a sport competition. Studies of animals, including birds, rodents, and primates, have shown that adult males experience an increase in their testosterone before a fight with another male. The psychological anticipation of competition or of an aggressive confrontation gets the body ready to give its best. Maintaining high levels of testosterone in the body all the time, however, is energetically expensive and has adverse health consequences because testosterone can weaken the body's immune system and make it more vulnerable to infectious diseases. Thus, our brains and bodies have evolved mechanisms to increase our testosterone when it's necessary and to keep it low when we don't need it.

Another context in which testosterone is necessary or helpful, at least for men, is sex. Testosterone can increase many aspects of male sexual performance. The psychological anticipation of sexual activity is generally accompanied by an increase in testosterone, again, to help the body give a good performance. Watching an erotic video clip makes a man think about sex and creates the psychological anticipation of sexual activity. It takes much less than watching porn, however, for a man to get ideas in his head. A 5-minute casual conversation with a woman he has just met can have the same effect.

Several years ago my former Ph.D. student James Roney, now a professor at the University of California Santa Barbara, conducted an experiment in which he asked a bunch of heterosexual male college students to come to our lab and take a personality test. Using an excuse, he left each student alone in a room with another young man or a young woman they hadn't met before, and who happened to be a research assistant in our lab. The research assistant had been instructed to have a casual conversation with the subject for 5 minutes and then leave the room. James collected saliva samples right before and right after the subject's conversation with the research assistant so that we could later measure his testosterone level. Lo and behold, the male students who chatted with the women showed an increase in their testosterone. The higher their testosterone the more they tried to impress the woman during the conversation (a new study conducted in The Netherlands and just published in the Proceedings of the Royal Society of London B showed something similar: male college students whose testosterone increased following a competitive computer task engaged in more courtship behavior during a subsequent brief conversation with an unfamiliar woman).

Whatever ideas our subjects got in their minds that day, we are pretty sure that nothing happened between them and our research assistants. However, if some of those young men went to the gym after visiting our lab, it's possible that they had an excellent workout.

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Cook, C. J., & Crewther, B. T. (2012). Changes in salivary testosterone concentrations and subsequent voluntary squat performance following the presentation of short video clips. Manuscript in press in Hormones and Behavior.

Roney, J. R., Mahler, S. V. & Maestripieri, D. (2003). Behavioral and hormonal responses of men to brief interactions with women. Evolution and Human Behavior, 24: 365-375.

 Van der Meij, L., Almela, M., Buunk, A. P., Fawcett, T. W. & Salvador, A. (2012). Men with elevated testosterone levels show more affiliative behaviours during interactions with women. Proceedings of the Royal Society of London B, 279: 202-208.

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