Most of us are right-handed–about 90 percent, in fact. Left-handed individuals account for only 10 percent of the population, and this ratio has remained consistent over time. The reasons are unknown. However, researchers at Northwestern University recently developed a mathematical model that may have the answer. According to their calculations, the low percentage of lefties results from an evolutionary balance between cooperation and competition. But what about pets? Is there paw preference in dogs and cats?