Evolutionary psychologists John Tooby and Leda Cosmides were puzzled by the fact that in the US, race is one of the three characteristics most often used in brief descriptions of individuals (the others are age and sex). They reasoned that natural selection would not have favoured the evolution of an instinct for using race as a classification, because for most of human history, humans almost never encountered members of other races. Tooby and Cosmides hypothesized that modern people use race as a proxy (rough-and-ready indicator) for coalition membership, since a better-than-random guess about "which side" another person is on will be helpful if one does not actually know in advance.