One nice natural experiment is drivers. People who drive in the US get more sun on the left side of their face and their left arm. People who drive in Australia

One nice natural experiment is drivers. People who drive in the US get more sun on the left side of their face and their left arm. People who drive in Australia get more sun on the right side of their face and their right arm. If you measure which side people tend to get cancer on it seems to track this. http://health.yahoo.net/experts/dayinhealth/shocking-evidence-left-sided-skin-damage http://health.yahoo.net/experts/dayinhealth/shocking-evidenc...