Lethal lefties!

I am slightly odd… ok, ok – I’m very odd, but I’m actually talking about something specific here. I eat my food with my knife in my left hand. As a proud left-handed person, and a very stubborn child, I insisted that if I was a true leftie I was going to eat my food left-handed as well. This now means that whenever I eat with other people I have to surreptitiously switch the cutlery round, and hope no-one notices! If they do, then there is the enviable slightly awkward explanation that I now find it impossible to eat with my knife in my right hand. This is usually followed by the person I’m with trying to cut with their knife in their left hand… only to exclaim that it’s just plain weird!

I think being a leftie is brilliant, any excuse to be a bit more different! But I wondered whether science could tell me if being left handed is good thing. So I did a very basic search – I typed in left-handedness into Google Scholar and saw what was thrown up. Worryingly, out of the top 20 or so results that were thrown up, it didn’t seem to be good news…

Innate left handedness and risk of breast cancer read the title of one BMJ study from 2005. The authors of this study had looked into the correlation between certain types of breast cancers and handedness. The motivations for examining this was that altered levels of sex hormones which someone is exposed to whilst in the womb may lead to left-handedness, and also change developing breast tissue into a source for cancerous cells. The data showed that, among other relationships, left-handed women were roughly twice as likely to develop premenopausal breast cancer then right handed women.

Inevitably this was picked up by mainstream media at the time; Breast cancer risk double for lefthanded women  was the title run by the Independent whilst the Mail Online went with the (rather subdued?) headline of  Left handers ‘more likely to get breast cancer.

The possibility of left-handedness being a result of altered hormone levels in the womb has also seen it studied in relation to other diseases. For example in 2009, in the Multiple Sclerosis Journal, a paper entitled The relationship between handedness and risk of multiple sclerosis suggested that hormone levels in the womb could not only result in left-handedness, but in an increased risk for MS as well.

It was with some trepidation that I opened the paper “Handedness, homicide and negative frequency-dependent selection

The study was another correlation study, so the conclusions drawn about causation may be slightly overconfident, but it is an interesting read. The abstract is actually a good summary of the paper:

Humans exhibit hand preference for most manual activities in which they are specialized. Right- and left-handers have coexisted at least since the Upper Palaeolithic, and left-handers are in the minority in all human populations. The persistence of the polymorphism of handedness is a puzzle because this trait is substantially heritable and several fitness costs are associated with left-handedness. Some countervailing benefit is required to maintain the polymorphism. Left-handers may have a frequency-dependent advantage in fights—the advantage being greater when their frequency is lower. Sports data from Western societies are consistent with this prediction. Here, we show that the frequency of left-handers is strongly and positively correlated with the rate of homicides across traditional societies. It ranges from 3% in the most pacifistic societies, to 27% in the most violent and warlike. This finding is consistent with a frequency-dependent selection mechanism maintaining left-handedness in these societies.

In simpler terms… studies have shown that us lefties are better as sport when there are fewer of us because right-handers find us harder to play against. The authors suggest that this might also be the case in a more violent context, in fights! They reasoned that if left-handed people are more successful in fights then they are also more likely to kill their opponent and so examined homicide date – the data showed a strong correlation between handedness and homicide!

The conclusion that was reached was that, if lefties are more successful in fights then, despite other disadvantages to being a left-hander, they are more likely to survive and not die out – thus explaining the maintenance of a small stable population of left-handed people in the community.

I told you I could be lethal with a lacrosse stick…!

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