Usually referred to as the nature vs debate, the debate over the relative contributions of inheritance and the environment is one of the oldest issues in both philosophy and psychology. Philosophers such as Plato and Descartes supported the idea that some ideas are inborn. Thinkers such as John Locke argued for the concept of tabula rasa - a belief that the mind is a blank slate at birth, with experience determining our knowledge, on the other hand. Most of the psychologists in our days believe that it is an interaction between these two forces that causes development. Some aspects of development are distinctly biological, such as puberty. The onset of puberty, however, can be affected by environmental factors such as diet and nutrition. The relative importance of early experiences versus those that occur later in life is a second important consideration that developmental psychology involves. Psychoanalytic theorists tend to focus upon events that occur in early childhood. Much of a child's personality is completely established by the age of five according to the Freud. Those who have experienced deprived or abusive childhoods might never adjust or develop normally if this is indeed the case.

Issues in developmental psychology

A third major issue in developmental psychology is that of continuity. Does change occur smoothly over time, or through a series of predetermined steps? Some theories of development argue that changes are simply a matter of quantity; children display more of certain skills as they grow older. Other theories outline a series of sequential stages in which skills emerge at certain points of development. Whether or not their child is developing normally is one of the biggest concerns of many parents. Developmental milestones can create concern when a child falls slightly behind the norm but offer guidelines for the ages at which certain skills and abilities typically emerge. Focus on individual differences in development is becoming more common while developmental theories have historically focused upon deficits in behavior. Traditionally psychoanalytic theories are focused upon abnormal behavior, so developmental theories in this area tend to describe deficits in behavior. Individual differences are an important component of these theories, so Learning theories rely more on the environment's unique impact on an individual. Psychologists look at both norms and individual differences when describing child development in our days. The theories are those influenced by the work of Sigmund Freud, who believed in the importance of the unconscious mind and childhood experiences. Freud's contribution to developmental theory was his proposal that development occurs through a series of the so called psychosexual stages. Learning theories, on the other hand, focus on how the environment impacts behavior. Important learning processes include classical conditioning, social learning and much more. Cognitive theories focus on the development of mental processes, skills, and abilities. Examples of cognitive theories include the theory of cognitive development written by Piaget.