Herbert Spencer, psychologist and philosopher, born in 1820. He stated that humans have a constant amount of energy that must be expended. Early in our existence, most, if not all, of that energy was used just meeting basic needs. As our civilization advanced, and less energy was used meeting these needs, we have had to compensate by expending our excess energy in some other manner, namely, play.
Sigmund Freud, psychoanalyst, born in 1856. He suggested that play was a way of expressing socially unacceptable behaviors. Play was therapeutic, allowing one to vent undesirable feelings and actions in a more acceptable manner.
Karl Groos, zoologist, born in 1861. He studied play first in animals, then in humans. He explained that play was a way of preparing for survival in the adult world. Maria Montessori, born in 1870, elaborated on this theory. She proposed that children would be better off if they spent their play learning, or imagining, useful things. These two theorists feel that "play is the child's work."
Jean Piaget, psychologist, born in 1896. His work focused on intellectual development in children, and his play theory reflects that. He suggested that human intellect develops in stages through assimilation (transforming the environment to meet the requirements of self), or play, and accommodation (transforming self to meet the requirements of the environment), or work.
Lev Vygotsky, psychologist, also born in 1896. His play theory emphasizes social development. He suggests that there is an ability level that children can reach but not without help from adults, which he refers to as a zone of proximal development, or ZPD. When children play, they give cues to adults about their readiness to learn new skills with assistance.
David Elkind, chair of the Department of Child Development at Tufts University, suggests that children play for personal, experiential reasons, and any developmental value is beside the point. In other words, they just want to have fun!