The Science of Goalkeeping

OK, this research is pretty silly, and quite frankly, I can't imagine what compelled the researchers to undertake it, but because it has to do with something I love, soccer, I feel compelled to blog about it. There this short report in the March issue of Psychological Science that I just got around to reading on goalies' influence of the direction of penalty shots in professional soccer matches. Masters et al.1 start with the recognition that goalies stop only 18% of penalty kicks. Given that goals are at a premium in soccer, with games frequently won by one goal (often the only goal of the game by either side), that's an amazing statistic. It's incredibly difficult to score on a goalkeeper in the course of a game, but it's pretty easy to do so with a penalty kick.

This made Masters et al. curious about what goalkeepers can do to influence penalty shots. So they watched clips of 200 penalty kicks from "World Cups, African Nations Cups, European Championships, and Union of European Football Association (UEFA) Champions League matches" (I, personally, think they just liked watching penalty kicks, and figured since they'd spent so much time doing so they might as well write a paper about it). They found that in almost all instances (96%), the goalkeeper stood just slightly (and I mean just slightly) off center, creating a difference between the distance of the goalie from the two goal posts of about 9.95 centimeters, which amounts to a difference between the areas to the right and left of the goal keeper of about 2.9% of the total area of the goal. The side to which the goalie stood did not, however, influence the side to which the goalie dove as the ball was kicked. So goalies didn't seem to be aware of their position. However, when they looked at whether penalty takers were aware of the position of the goal keeper, they found that 103 out of 174 (I'm not sure what happened to the other 26 kicks) were to the side of the goal keeper with more space. So the position of the goal keeper does appear to affect the direction of the kick on a (statistically) significant percentage of penalty kicks. Penalty takers are, then, aware of the goalie's position.

To follow up their observational data, Masters et al. conducted three laboratory studies. In the first, they had participants view a scaled version of a goal on a computer screen, with a block (representing the goal keepre) placed either at different distances to the right or left of the center of the goal. Participants were asked to indicate to which side of the goal's center the block stood, and to report their confidence in their judgment. Their judgments were above chance (above 50:50) at differences in the total area on either side of the block of as small as 0.5%, despite the fact that at such small differences, the participants showed little confidence in their judgments. So it seems that people are able to notice such small diversions from center, but that they're unaware that they're noticing them.

In their second study, they used a much larger scale model of a goal (44% of the size of actual goals), and placed an image of an actual goalie (Oliver Kahn, the guy in the photo above) either to the right or left of center. This time, participants were given a ball on a "penalty spot" located about 5 meters from the screen (which, obviously, was no ordinary computer screen), and were told to kick the ball at the goal. Once again, at differences in the area to either side of the goalie of as small as 0.5%, participants shot at the side with the greater area significantly more often (i.e., above chance) than to the side with less area, though participants did not consciously recognize those differences in area until they were as large as 3.0%.

In a third study using the same stimuli as the second, they got similar results. This time, participants were asked to shoot only when Kahn was in the exact center of the goal. Consistent with the evidence from the first two studies that participants are unaware of the goalie being off center, participants tended to shoot even when the differences in area between the two sides of the goalie were as large as 3.0%, and as in the previous study, they tended to kick the ball to the side with the larger area, even when the differences were as small as 1.6%.

So what does this study teach us? Well, not a damn thing, really. Here's how Masters et al. ended their paper:

In summary, we have shown that it is feasible for a goalkeeper to influence perceptions of area and consequently the direction of penalty kicks by standing marginally to one side or another of the goal center; the goalkeeper can then strategically dive to the side with greater area. Extrapolation of our data indicates that the optimum displacement of the goalkeeper in real life is from 6 to 10 cm. The penalty taker is unlikely to be mindful of a displacement in this range, but is at least 10% more likely to direct the penalty kick to the side with greater area than to the side with smaller area.

Yeah, a lot of good that knowledge does us. If we were soccer coaches, and wanted to use this research, we'd quickly find that it wouldn't help us a bit. Kickers are obviously already utilizing their ability to detect to which side of center the goalie is standing, and goalies seem to be unable to notice their position relative to the center, so they can't use it to decide in which direction they should dive. But hey, some psychologists got to watch a bunch of penalty kick videos, and I got to write a post about it, so it's not a complete waste.

¹Masters, S.W., van der Kamp, J., & Jackson, R.C. (2007). Imperceptibly off-center: Goalkeepers influence penalty-kick direction in soccer. Psychological Science, 18(3), 222-223.