Developmental psychologists describe, explain and attempt to predict age-related behaviors.
Temperament seems to have a strong biological basis as it tends to be stable over time. Two important stage theorists of psychological development are Sigmund Freud and Erik Erikson. Each period has a sexual focus that is gratified, frustrated or overindulged, leaving a lasting imprint on personality. . As he attempts to gain more control he derives pleasure from exploring the environment, testing the strength of his will power over parents and authority figures. A crucial event to Freud was toilet training that represents society's systematic efforts to control the child's biological urges.