American psychologist Peter Gray hypothesizes that band hunter-gatherer societies are able to reduce aggression while maintaining relatively peaceful, egalitarian relations between members through various methods, such as fostering a playful spirit in all areas of life, the use of humor to counter the tendency of any one person to dominate the group, and non-coercive or "indulgent" child-rearing practices. Gray likens hunter-gatherer bands to social play groups, while stressing that such play is not frivolous or even easy at all times. According to Gray, "Social play—that is, play involving more than one player—is necessarily egalitarian. It always requires a suspension of aggression and dominance along with a heightened sensitivity to the needs and desires of the other players".