The argument runs like this. In prehistoric times, humans were subject to both natural and sexual selection. Sexual selection works in two ways. Firstly, there is mate choice (intersexual selection), which may be expressed by one or both sexes - i.e. either men or women or both may select their mates according to certain criteria of judgement. Secondly, within a sex (usually males) there may be competition for resources, particularly those which enable access to mates or successful child-rearing. This second form of selection, called intrasexual selection, is what produces fighting between the males of many species. Many Western societies now frown upon direct competition in terms of fighting, but the characteristics which make for good fighters likely remain - and may be expressed as prowess in sports which require high levels of spatial judgement, speed, endurance and strength. Football may be one such sport.
If this was the case, we might expect to see correlations between the levels of the male hormone testosterone, often associated with strength and other typically male characteristics, and football ability. Testosterone has also been linked to the formation of an efficient cardiovascular system in men, making it potentially even more important for fighting and/or football playing, although its action upon these systems occurs before birth (pre-natally), and therefore cannot easily be measured for large samples of football players.
To test for a link between football (or sporting) abilities and testosterone levels, then, researchers have to be a little more creative. The paper I have recently read on the subject, published in 2001 by Manning and Taylor, for instance, looks for correlation between sporting abilities in football players of various standards and the ratio of the second digit (the index finger) and the fourth digit (the ring finger). This ratio, written as 2D:4D, is typically lower in men than in women. That is, men tend to have shorter index fingers relative to the length of the ring finger while women have the opposite. This ratio has been explicitly linked to testosterone levels during foetal development, and does not change after a child is born (barring accidents involving the fingers), making it a good proxy for prenatal testosterone levels (Manning and Taylor 2001).
Manning and Taylor, therefore, carried out three studies looking for a link between the 2D:4D ratio, sporting ability (particularly in football) and visual-spatial judgement, thought to be an indicator of high "fitness" in men. The first two of these studies used participants from sports centres and libraries, and asked them to rank their sporting abilities on a scale from 10 ("I have represented my country") down to 0 ("I do no sport"). The first of these studies found a link between 2D:4D ratios and sporting scores such that the higher the score a participant gave the lower the digit ratio was, and hence the higher their pre-natal testosterone exposure likely was. The second found a similar link between visual-spatial judgement scores and 2D:4D ratios. Both these relationships were quite variable (with participants at a particular sporting level having varying digit ratios), but were statistically significant, that is, highly unlikely to have arisen due to chance alone.
The third study, meanwhile, examined footballers specifically and ranked them according to their league and coach references. It also involved a "control" group of non-footballers, for comparative purposes. It found that there was not only a difference in 2D:4D ratio between footballers and controls (footballers had lower average digit ratios), there was also a decrease in digit ratio the higher up the sporting scale the footballer was. So international players had lower 2D:4D ratios than players in the premier league, who had lower ratios than first division club players, and so on. Coaches, interestingly, fell between internationals and premier club players, suggesting (as is indeed the case) that they would have been highly successful footballers themselves (Manning and Taylor 2001).
So, all this suggests that footballers, and sporting professionals in general, are successful because they are some of "the fittest" in an evolutionary sense; they have high pre-natal testosterone levels, and hence well developed "fighting" skills which can be transferred into sports. Manning and Taylor also note that there are two possible explanations of this high fitness. Firstly, as study two suggested, there may be a link between visual-spatial awareness and pre-natal testosterone levels. Alternatively, or as well, the role of testosterone in the development of the cardiovascular system may be important. Both hypotheses are supported by some evidence (for example, that exposure of male foetuses to female hormones in the womb can lead to both digit anomalies and malfunctions of the cardio-vascular system), but we cannot yet discriminate between them. Still, if the great social institution and economic phenomenon that is international football could have arisen as the result of selection for male fighting abilities, we may be looking too hard for direct evolutionary explanations of other modern human traits like culture and language. Perhaps they, too, as some researchers suggest, were in part the by-products of selection for other features.
References
Manning JT, & Taylor RP (2001). Second to fourth digit ratio and male ability in sport: implications for sexual selection in humans. Evolution and human behavior : official journal of the Human Behavior and Evolution Society, 22 (1), 61-69 PMID: 11182575
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