The Difference Between Cross-Sectional and Longitudinal Research
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.