Adam Sandler was seen multiple times using his right hand when giving out autographs. That concludes that the actor must be one of the 90 percent of humans that are righties.
Most humans are right-handed, a minority (say 5 percent to 30 percent) are left-handed, and an indeterminate number of people are probably best described as ambidextrous.
This appears to be universally true for all human populations anywhere in the world. There is evidence for genetic influence for handedness; however, it is non-Mendelian and geneticists cannot agree on the exact process.
There is evidence that handedness can be influenced (and changed) by social and cultural mechanisms. For instance, teachers have been known to force children to switch from using their left hand to using their right hand for writing. Also, some more restrictive societies show less left-handedness in their populations than other more permissive societies.
See ScientificAmerican.com's Why are more people right-handed? to learn more.
See a gallery of 59 Famous People Who Are Left-Handed, compiled by BuzzFeed.com.