3.1 Themes and Methods in Developmental Psychology

Developmental psychologists look at change across the whole lifespan, not just childhood. Some focus on the order of development, describing what typically happens at each age. Others focus on themes that cut across all ages: physical growth, thinking, language and relationships. Units 3.2 through 3.6 take each of these areas in turn.

Most psychologists now take a middle position on all three: nature and nurture both matter, some abilities develop in rough stages while others change gradually, and personality shows both stability and change.

• Nature and nurture: how much of development comes from heredity and how much from experience, and how they interact (1.1). For example, a child's language ability depends on a brain ready to learn language and on hearing language around them.
• Continuity and discontinuity (continuous versus discontinuous development): is development a smooth, gradual climb, like a ramp, or a series of distinct stages, like a staircase? Stage theorists such as Piaget (3.4) and Erikson (3.6) see stages; others see gradual change.
• Stability and change: do traits stay the same over time, or do people change? Temperament, a baby's basic emotional style, shows some stability into adulthood, but people also change a lot, especially in how they handle emotions and relationships.

A cross-sectional study compares people of different ages at the same point in time. To study memory and aging, a researcher might test groups of 20-, 50- and 80-year-olds this month. It's quick and inexpensive, but it has a major weakness: the groups differ in more than age. People born in different eras grew up with different schooling, technology, nutrition and health care. A group of people born around the same time is called a cohort, and differences caused by when people grew up are called cohort effects. They can be mistaken for effects of aging.

A longitudinal study follows the same people over a long time, testing them again and again. A researcher might test the same group at ages 20, 50 and 80. This shows how individuals actually change and avoids cohort effects, because everyone grew up in the same era. But it's slow and expensive, and participants drop out over the years (called attrition). If the people who drop out differ from those who stay, the results can be biased.

The two designs can give different answers. Early cross-sectional studies suggested that intelligence declines steeply in adulthood. Longitudinal studies following the same people found much smaller declines until late in life. Part of the difference was cohort effects: older groups had, on average, less formal education than younger groups, so they scored lower for reasons that had nothing to do with aging. Both designs are correlational, because age can't be randomly assigned.