Thursday, 31 May 2018

The Evolution of Sex Differences in Throwing

One of the most robust sex differences occurs in throwing. Men can throw (on average) much faster and therefore much farther than women, and this gap even exists at comparable levels of sports such as baseball and softball. The most common explanations are that a) men are, on average, larger and stronger than women, and b) most cultures gender throwing activities as male, leading to earlier acquisition and much more practice. YouTube has plenty of videos of men throwing with their off hand that point to the critical role of learning. 

However, Lombardo & Deaner (2018; L&D) have just published a hypothesis that while these factors are at play, they rest on top of an underlying biological advantage and that 'throwing is a male adaptation'. Specifically, they claim that there has been greater selective evolutionary pressure on men (as compared to women) to develop the strength, skills and anatomy needed to throw for large distances and great accuracy. Men have evolved to be better throwers than women.

This post will briefly review the hypothesis and the evidence, and then come to two conclusions. First, many of the differences they discuss seem quite closely aligned to the cultural sex differences around throwing that we know exist and so may not be biologically innate. Second, and more importantly, there may not even be a throwing-specific sex difference to explain. Right now, the only clear finding is that men throw faster; but they are also (on average) stronger and larger for non-throwing reasons. There is, as yet, no clear evidence that men are better throwers. I will then review some recent data of my own that suggests when the full perception-action task dynamic is analysed in closer detail, trained women show every sign of being equally skilled throwers as trained men.

The Hypothesis

L&D first consider the origin of throwing. Modern humans have been anatomically able to throw well since Homo erectus, but at some point we were non-throwing primates. What made us begin to develop along a trajectory that enabled throwing? (NB the following is their argument, not mine; I'll note when I'm interjecting a comment)

L&D note that modern non-human primates never throw in hunting contexts (ADW note: because they can't, because they live in crowded jungles where throwing isn't that useful, or because they are often not primarily carnivores - take your pick). When they do throw, they do so in the context of antagonistic social encounters, to scare off a same-species competitor. Most of this kind of behaviour is done by the males. So throwing has, since the dawn of the skill, been a primarily male behaviour. 

Once we became the 'primates who throw', speed and accuracy went up and this skill showed up in hunting and warfare (all mostly male activities). Males who were better at hunting and warfare got more and better mates and more and better children. This feedback loop kept the highest pressure on males, and thus the sex differences in modern human performance are the result of throwing specific, male-selective evolutionary pressures. 

The Evidence 

Behavioural

Boys throw very early and much earlier than girls, and 'early' suggests 'biologically innate' over 'socialised'. While girls/women can learn to throw, their speed and form never catch up to men's. Finally, men are better throwers overall, where 'better' means they can throw faster and farther, and more accurately (although the data here are very limited). These differences persist cross-culturally, even when female throwing is more normal, or when things like the American obsession with baseball isn't a massive driver, or when the women are highly trained like the men.

They also discuss male advantages in interception, but of course interception and throwing are two entirely different perception-action task dynamics and they are therefore not related in the way the authors assume. 

Anatomical

L&D review pectoral girdle anatomy and show some of the features that support throwing show sex differences in favour of men. Some do not, and some go the other way; some of the features are affected by use and some are less so. Only some are clearly throwing specific, and others may have become male-advantaged or female-disadvantaged for non-throwing related reasons. 

Analysis

As I summarised in the Introduction, most of the evidence is suggestive but not clearly much more. The least contestable difference is in throwing for aggression, hunting and war (although I'd bet the story is muddier than they think). But this cultural difference will only make throwing a male adaptation if it shows up as male-specific anatomical or behavioural differences. The behavioural differences all have a confound, namely the culturally-driven high male involvement in throwing. The authors admit as much (pg 110) but don't take it seriously enough. The anatomical differences are often there, but use (biased by the cultural gendering of throwing) and non-throwing related selection pressures remain options for many of them. 

My primary concern, however, is with the overall framing; that men are better throwers than women and that this needs explaining. 'Better' mostly means 'faster', and they have made no effort to parcel out the contribution to this of the non-throwing related strength and size differences between men and women they review on page 93. 'Better' briefly means 'more accurate', although the evidence is only two unconvincing papers (page 97). Finally, 'better' occasionally means 'throwing form', although this is only ever assessed using an observational checklist looking for a particular, mature form; no detailed kinematic analyses and no understanding that action control is not about the production of one perfect movement. Overall, therefore, they present very little evidence that men are 'better' throwers than women. 

A Little Data

I actually have some data that I plan to dig back into to test the hypothesis that men are better throwers than women. It will take a little while to get the formal analysis done (I need to get Sternad's T-N-C analysis code working on my solution manifolds) but I can demonstrate the idea with a couple of graphs.

Wilson et al (2016) tested three groups of skilled throwers trying to hit a 4ft x 4ft target at 3 distances (5m, 10m and 15m) and 3 heights (centre at 1m, 1.5m and 2m). The three groups were male American college level baseballers, female American college level softballers, and male UK club level cricketers. 

The baseballers threw about 10m/s faster than the softballers, but the softballers threw about 10m/s faster than the cricketers and at around 30m/s. So the data showed a clear difference as a function of sport, with the women actually in the middle and throwing fast. 

However, there are two elements in the data that go beyond speed as a metric for quality; refer to Figure 1. 
Figure 1. Release parameters from Wilson et al, 2016
First, note that within each group, everyone scales their release speed in basically the same way as distance increases (Figure 1a). Second, note that release angle varies appropriately within every group as a function of distance (to offset the speed differences) and of height (to maintain a flat, fast trajectory). So the softball women, while not the fastest group, showed a lot of evidence for the same level of control over the relevant action parameters.

The other part of that paper was using affordance maps (solution manifolds) to assess performance. This involved simulating throws across a wide range of release parameters, identifying which combinations produced hits, and placing the human data on that map. I've produced an example of softball performance in Figure 2
Figure 2. Female release speed and angle combinations mapped onto the solution manifold/affordance map
As you can see, the females produced very consistent data that lives within a stable region of the solution manifold. This tight clustering and good location was typical across all conditions; in fact, the softball data was much tidier than the men, who tended to be spread out along the release speed axis. I interpreted this as the men trying to throw as fast as possible, not always getting it right, and making the necessary adjustments. Good control, just a messy strategy. So along this dimension, I'd argue the women are throwing better!

I can now quantify these patterns using Sternad's Tolerance-Noise-Covariance analysis, and I will do this comprehensively for this data as soon as I have time to get her student's code working. These three components assess three different aspects of execution, relative to the solution manifold, and I am guessing that they will show the women are producing at least equally skilled throws as the men. I think I will also get into this more explicitly in the future (Registered Report, anyone?)

My current conclusion is that the evidence for L&D's strong evolutionary hypothesis is, in fact, weak, and that it's time to get into this with up-to-date perception-action techniques (and probably some more detailed evolutionary biology). 

References


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