Gavin Pretor-Pinney, author of The Wave Watcher's Companion found out:
It was not the most robust study, I admit, but I did watch 94 different YouTube videos of Mexican waves. (Looking back at it, this was clearly a displacement activity designed to avoid getting on with writing my book.) Sixty-nine of them were of waves travelling around stadiums within the Northern Hemisphere. Of these, I counted 40 going clockwise and 29 counterclockwise, a ratio of 58:42 in favour of clockwise. The other 25 videos were of waves at games in the Southern Hemisphere. Of these, 10 waves went clockwise and 15 counterclockwise, a ratio of 40:60 in favour of counterclockwise.
I asked a professional statistician if these results were significant. She told me that I can be 96.6 per cent certain that the probability of a wave going one way rather than the other is different between the Northern and Southern Hemispheres. There is a high probability of a hemispherical difference. It certainly looked like waves are more likely to go clockwise in the north and anticlockwise in the south, but I hadn’t watched enough videos to be able to say this with the 95 per cent certainty that is considered significant.
http://themexicanwaveexperiment.org/node/20
http://www.snopes.com/science/coriolis.asp
Break this "study" down into individual nations and see if there is a correlation with which side of the road the nation in question drives on.
When you start a wave at a stadium you usually make eye contact with the people in the direction which you expect the wave to travel. Being from the U.S. I would be inclined to look to the left (clockwise). The head/body motion involved in starting a wave is the same one I'm used to doing dozens of times a day; lean forward, rotate head 90 degrees. In a country where we drive on the right side of the road we look left, both first and most often.
When looking at a clock we tend to think of clockwise as moving to the right even though the hands are also moving to the left for the same amount of time. I think that on a horizontal plane, directional preference is probably just a matter of what we're most used to doing.
And of course, as BJN said, the Coriolis Effect has no influence.
That, I'm afraid, depends more on the shape of the sink than the location of the sink
It is fun to make the wave thou.