Because left-handed people have different brains.
Of course, everyone has different brains, the issue in studies is whether or not the difference is meaningful and can easily be controlled for in data. The authors of a new paper in Nature Reviews Neuroscience contend that left-handed people are rarely included in neuroscience studies, not because they are only 10 percent or less of the population, but because the differences with right-handed people cause noise in the final results.
Neuroscience studies overall suffer from suspect methodology and poor grasp of statistics and if the handedness of participants is skewing results, that's even worse. If researcher can't control for that, they certain can't control for sexual orientation, gender or anything else where brains may be different.
The authors say more left-handed representation would add valuable information about brains and genes as well as about several psychiatric disorders.
Verbs that express actions ('to write', 'to throw') have opposite effects in the motor cortex of left-handed and right-handed people. This figure is taken from the article published in Nature Reviews Neuroscience on 12 February 2014. http://www.nature.com/nrn/journal/vaop/ncurrent/pdf/nrn3679.pdf
"One of our studies from 2009 clearly shows why research into left-handed people is so vital," says Willems. "According to the textbooks, facial recognition takes place in the right half of the brain. Our research revealed that the same process takes place in both halves of the brain in the case of left-handed people, but with the same final outcome. That is a fundamental difference. And left-handed people might process other important information differently as well. The minimal amount of research into this is, in my view, a missed chance for the neurosciences."
Data from left to right
According to Willems, the same applies to genetic research.
Schizophrenic and psychotic patients are more likely to be left-handed. Up until now little has been done with that information to clarify the genetic links with the disorders concerned.
Left-handed and right-handed people perceive actions in different ways. Left-handed people do that with the right half of the brain and right-handed people do that with the left half of the brain. This figure is taken from the article that Willems and colleagues published in 2009 in Frontiers in Human Neuroscience. http://www.frontiersin.org/human_neuroscience/10.3389/neuro.09.039.2009/full
"Databases without left-handed people are not representative for the population and in view of the large number of genetic databases currently being set up, ignoring left-handed people is not wise," says Willems. The researchers are setting up a website, www.mpi.nl/handedness, to encourage left-handed people to participate in research.
Citation: Roel M. Willems, Lise Van der Haegen, Simon E. Fisher&Clyde Francks, 'On the other hand: including left-handers in cognitive neuroscience and neurogenetics', Nature Reviews Neuroscience 2014 www.nature.com/nrn/journal/vaop/ncurrent/pdf/nrn3679.pdf