The principal theorists to consider are Freud, Erikson, Piaget, Ainsworth, Vygotsky, Skinner, Pavlov and Bowlby. Piaget and Vygotsky will be investigated in more detail later, but are included here in order to provide an overview of what brought us to this moment in developmental theory.

Freud began the study of child development by looking at childhood as a progression through a series of psychosexual stages. As the name suggests, this theory focuses on the development through sexual stages and forms of pleasure. This was further developed by theorists such as Erikson (1998). Erikson postulated that there were seven stages of development, extending into old age, and each of these stages was defined by a central conflict that had to be resolved before development of the self could progress. Collectively, theories such as this that focus on the mind are psychoanalytic, and fall on the nurture side of the debate.

In the centre of the debate are thinkers such as Ainsworth (1991), Vygotsky (1987) and Piaget & Cook (1952). These theorists argue that there is a regular series of developmental stages that children must pass through, and the particular experiences they have during these stages define the outcome of their development.

Finally, there are the behaviourist thinkers such as Skinner (1938) and Pavlov (1927). These thinkers argue that everything is the result of conditioning, and that children's development depends entirely on their behavioural conditioning - they believe that nurture is everything, regardless of the particular nature of children.

These differing stances have a substantial impact on all child development research, since most research tends to occur within one of the traditions of thought, therefore changing the results that are seen and how they are interpreted. Each of the stances has some evidence, but there is no clear understanding of which is most correct. Modern approaches tend towards systems and genetic theories, assuming that development is an action within a system that includes the child and environment as factors that interact, but are different in different specific behavioural and developmental instances.