This research method builds on both the correlational and experimental methods. The focus is on how we grow the become who we are. Since we are not born with complete adult capabilities, the developmental approach looks at all of the contributing factors to our growth.
There are two experimental designs that are typical of developmental research. Children can be measured over their lives. This longitudinal design requires a huge investment in time (years) and in resources (money). It’s expensive to measure people every year for 20 or 30 years. Subjects are not always available, move away, or become uninterested. Researchers move on to other topics, and in some cases get old and die.
The other alternative is a cross-sectional design. Instead of following the same people over time, people from several age groups are tested at the same time. Children, teens, adults and the elderly can be tested on the same day, using the same materials. This method is fast, efficient and cheap.
The downside is that we are guessing that these children will grow up to be like these teens. And that these teens will grow up to be like these adults. Researchers compromise by doing the cross-sectional studies first and following up with an occasional longitudinal study.
Longitudinal studies often start with a general concept or principle, like intelligence, and try to reduce it to a simple explanation. In contrast to this deductive method, cross-sectional studies are often inductive. They gather lots of data and try to discover a general principle.