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Developmental psychology focuses on how our psychological characteristics change and develop throughout life, from birth (or, indeed, conception) to old age. Developmental characteristics studied include personality, development of relationships with others, cognitive capacities, and biological changes. All these characteristics are seen as interacting: for example the level of biological development influences our cognitive capacity; this will in turn have consequences for social interaction and so on. Two of the research designs used in developmental psychology are:

• longitudinal studies, Where the same people are followed over time, and their changes in behaviour plotted.
• Cross-sectional studies – which look at different people in different age groups, examing their different capacities in terms of cognition, capacity for social relationships etc.

One of the first major influences on developmental psychology was Darwin, whose theory of evolution prompted a radical re-examination of the way people thought about human development. The first idea that developmental psychology inherited from Darwin's theory was a functionalist perspective, arguing that if a behaviour is functional it increases the organism's chances for survival. Darwin examined the high degree of similarity between adults and children within any one species, as well as degrees of difference, suggesting that some actions must be innate reflexes rather than learned behaviours. An individual is the result of a gradual sequence of prior changes, both in a broad evolutionary sense and within that individual's own lifetime. Throughout, an individual's life further development and change lies ahead. This emphasis on gradual and continual change forms the basis of modern lifespan psychology. At the beginning of the 20th Century developmental psychologists were particularly concerned with charting the ages at which certain changes in behaviour 'normally' occur (e.g. when does a child talk for the first time). This developed into an approach to studying human development that is known as the organismic approach. That is, the individual (or 'organism') is its main focus. Changes in behaviour throughout life are typically presented as a natural sequence of changes that occur sequentially, in a fixed order, so that an individual has to pass through an earlier stage before reaching a later one. This is referred to as a stage theory. It should also be noted that there are distinct 'developmental lines' which tend to develop in parallel, for example cognitive development (cf Piaget) and psychosexual development (cf Freud). One of the assumptions of an organismic stage theory is that environmental influences, while important, can only affect the speed of development. They may slow development down, or accelerate it, or even stop it, but they cannot alter the nature of the stages themselves or the sequence in which they occur.

An early influential example of such an approach to human development was Freud's theory of psychosexual stages, which a child had to pass through, seen as central to the emotional development of that child. If a crisis occurred during any developmental stage, then it would be reflected in that person's personality. Like many earlier developmental theories, it tended to conceive of adulthood as a relatively static 'product' of childhood development, rather than seeing it as having its own unique developmental stages. Another developmental approach in the organismic tradition was produced by Piaget, who focused on cognitive development rather than emotional development. Although Freud and Piaget both took an organismic approach, their research methods and psychological traditions are very different. Freud's ideas were based on the clinical data of psychoanalysis, whereas Piaget's theorizing was based, initially, on observation of his own children - informed by biological, evolutionary and psychometric perspectives.\nIn contrast to the organismic approach, the mechanistic approach to representing human development focuses not on the individual, but on his or her behaviour. Development is primarily seen as the product of environmental influences (external factors), with genetic inheritance and cognitive processes (internal factors) seen as less significant. This approach draws on behaviourism and its principles, where development is characterised as a sequence of behavioural responses to environmental stimuli. However, behaviour analysis attempts to address the full context and complexity of human responses to his or her environment, and differs from behaviourism is in its greater acknowledgement of biological influences and constraints on development. \nInitially, developmental psychology focussed primarily on changes during childhood; however, 'lifespan' theories (such as Erik Erikson's) attempt to look at possible developmental challenges occurring throughout adult life also. This represents the beginnings of attempts to overcome the rather pessimistic view implied by traditional approaches, which viewed adulthood as a relatively static 'end-point' of childhood development, with the only change perceived as the physical and psychological decline of old age.

Erik Erikson's lifespan psychology was pioneering in several ways: firstly, in explicitly recognising that psychological development continues during later life, and trying to map some of the key transitions. Secondly, because of its emphasis on the relationship between the individual and society in affecting personal development. Erikson's model of later life is essentially person-centred. Another approach is function-centred: looking at just one type of behaviour (e.g. memory) and measuring variation in it across the lifespan. This approach focuses on changes in ability with age. These changes can include both losses and gains (e.g. loss of processing speed compared to a young adult, but potentially balanced by increase in experience and knowledge). Modern developmental psychologists increasingly conceive development is as a transaction between the individual and their environment, with each influencing the other and in turn affecting the developmental path followed. This acknowledgement of the role a person plays in determining their own environment (and vice-versa) means that the 'nature-nurture' distinction is seen as overly simplistic in developmental psychology. Modern theories also conceive development as the outcome of interactions between age-related factors (e.g. biological maturation, social events such as attending school), historically-related factors (e.g. evolution, the occurrence of war), and 'random' biological and environmental occurrences that only relate to one individual. This approach to lifespan development is known as 'contextualism', drawing on a wide range of perspectives in addition to psychology, such as neuroscience, sociology, history and anthropology. The basic idea behind developmental contextualism is that development does not occur in isolation, it is affected by the context of a person's life. It is suggested that internal influences on development like an individual's biology and psychology interact with external factors such as their cultural influences, interpersonal relationships etc. It is this interaction between influences that results in human development. With this approach, development is seen as clearly embedded within society, its cultures and history.