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Cross-Sectional Studies

On the other hand, cross-sectional studies cost less, do not lose participants, and produce results quickly, but have other major weaknesses. In a cross-sectional study, researchers assess developmental changes with respect to a particular factor by evaluating different age groups of people at the same time. For example, to study lifespan changes in mathematical skills, psychologists could give the same math tests to groups of 15-, 25-, 35-, 45-, 55-, 65-, and 75-year-olds at the same time. Cross-sectional studies can be invalid if a cohort, group of people in one age group, is significantly different in their experiences from other age groups, resulting in the cohort effect, differences in the experiences of each age group as a result of growing up in different historical times. This difference is a confounding variable in the study. Obviously, most younger participants may have been exposed to calculators and computers their whole lives, whereas 65- and 75-year-olds have had fewer opportunities.

Cohort-Sequential Studies

To minimize the major drawbacks of both longitudinal and cross-sectional research designs, some researchers conduct cohort-sequential studies. In cohort-sequential studies, cross-sectional groups are assessed at least two times over a span of months or years, rather than just once. Results from one cohort are compared with other cohorts at the same age to evaluate their similarity; differences indicate a cohort effect. In this way, researchers can separate age-related changes from cohort effects. These studies share disadvantages of longitudinal research, but to a lesser extent.

Retrospective Studies

Biographical or retrospective studies are case studies that investigate development in one person at a time. Typically, a researcher interviews an individual at the older end of the age span of interest. The researcher reconstructs changes that have occurred in the subject’s life through the subject’s self-reports in interviews and examination of available data. Although these studies can be very detailed, they are not always correct because memory is not always accurate, and they may not be generalizable to a larger population.

Physical Development

Physical development focuses on maturation and critical periods. A critical period is a time interval during which specific stimuli have a major effect on development that the stimuli do not produce at other times.

Prenatal Development

Prenatal development begins with fertilization, or conception, and ends with birth. The zygote is a fertilized ovum with the genetic instructions for a new individual normally contained in 46 chromosomes. (See Behavioral Genetics in Chapter 7.) During the first 2 weeks following conception, the zygote divides again and again forming first a hollow ball of cells that buries itself in the wall of the uterus, then a three-layered inner cell mass surrounded by outer cells attached to the uterine lining. Different genes function in cells of the three different layers; the forming individual is now considered an embryo. During the embryonic period from the third through the eighth weeks following conception, organs start to develop as a result of differentiation and specialization of cells; and the placenta, umbilical cord, and other structures form from the outer cells. As organs develop, the embryo is particularly sensitive to environmental stimuli such as chemicals and viruses. Nutrients, oxygen, wastes, and other substances pass from the woman’s blood into and out of the developing organism through the placenta. By the end of the eighth week, the embryo has a head with partially formed eyes, limbs, and a skeleton composed of cartilage. At this point, all organs are present in rudimentary form, and the developing individual begins to resemble a human; it is a fetus, the developing human organism from about 9 weeks after conception to birth. During the fetal period, the organ systems