Studying Lifespan Development

Scientists study the development of humans across the lifespan by using two types of studies: cross-sectional and longitudinal. Cross-sectional research compares people across different age groups on a variable of interest (the concept or item a study seeks to measure). For example, developmental researchers may compare the language skills of children who are currently two, six, and ten years of age. Longitudinal research follows a single group of people over time, studying those individuals as they age. For example, a researcher may study how language skills develop in a single group of children, evaluating those children multiple times from birth to five years of age.
These research designs allow scientists to isolate the effects of nature (genetic factors) versus nurture (environmental factors). Nature and nurture each drive development through a complex, bi-directional interplay between genes and the environment. For example, a researcher who is interested in language may compare children growing up in bilingual families to those growing up in monolingual families. Children's language skills would be compared at the beginning of the study (initial cross-section) and then over time (longitudinal). This would allow the researcher to compare the effects of maturation and language exposure (one versus two languages) on the children's language development.

Some cognitive skills are characterized by a critical period, a time-limited developmental period during which the brain is highly sensitive to particular experiences. For example, if children are not exposed to language at all during the first few years of life (such as in severe cases of abuse or neglect), they do not develop a full capacity for language. Later exposure to those experiences cannot make up for the earlier loss. An extreme example is that of Genie Wiley, a girl born in California in 1957. Genie was kept in isolation with nearly no social interaction or care from approximately 20 months of age until she was 13 years old. Her father suspected that she had an intellectual disability when she was an infant and refused to care for her. She was kept alone in a room, almost always strapped into a crib, and received no social or emotional stimulation. When Genie went into state care around 13 years of age she had limited social skills and had not yet learned a language. Although interventions helped her learn the meaning of many words, she had difficulty with grammar and never learned to speak fluently. Tests indicated that she was processing speech in atypical areas of her brain, likely because she had missed the critical period for learning language.

People born at roughly the same time are called a cohort. In a research study, a cohort effect occurs when apparent developmental differences between groups are actually due to experiential differences rather than age differences. For instance, in lifespan studies of intelligence, older adults often score lower on tests of knowledge than younger adults. Yet concluding that general knowledge declines with age would be a mistake. Due to generational changes, older adults have fewer years of education than younger adults. These age-related differences in general knowledge do not necessarily reflect true age-related declines.