Historically, there has been some oppression of left-handed people in the Western civilization, from people thinking that left-handedness is the work of the devil to teachers forcing left-handed students to write with their right hand. However, the world still isn’t always friendly to lefties.
Northwestern University psychology professor Robin Nusslock told the Wall Street Journal that many studies about how the brain works specifically prohibit left handed people from taking part because their brains are wired differently than right-handed people’s.
Researchers want their sample to accurately represent the majority of the population and think that lefties would screw up the results. However, this means that many new breakthroughs in understanding how our brains work cannot be applied to 10% of the population. The same exclusion used to apply to women until President Clinton signed an act requiring that they be included in clinical trials.
Personally, I had always thought that psychology experiments were supposed to give us insight about a particular group of people and thus had to test each sub-group in that group in order to do so; if 10% of people are left-handed, then 10% of the sample being experimented on should be left-handed. It just seems right.