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Developmental scientists who seek to test hypotheses about causal relationships, such as whether media exposure influences behavior or whether hearing particular types of music influences mood, employ experimental research. An experiment is a procedure that uses control to determine causal relationships among variables. Specifically, one or more variables thought to influence a behavior of interest are changed, or manipulated, while other variables are held constant. Researchers can then examine how the changing variable influences the behavior under study. If the behavior changes as the variable changes, this suggests that the variable caused the change in the behavior.

For example, Gentile, Bender, and Anderson (2017) examined the effect of playing violent video games on children’s physiological stress and aggressive thoughts. Children were randomly assigned to play a violent video game (Superman) or a nonviolent video game (Finding Nemo) for 25 minutes in the researchers’ lab. The researchers measured physiological stress as indicated by heart rate and cortisol levels before and after the children played the video game. Children also completed a word completion task that the researchers used to measure the frequency of aggressive thoughts. Gentile et al. (2017) found that children who played violent video games showed higher levels of physiological stress and aggressive thoughts than did the children who played nonviolent video games. The researchers concluded that the type of video game changed children’s stress reactions and aggressive thoughts.

Let’s take a closer look at the components of an experiment. Conducting an experiment requires choosing at least one dependent variable, the behavior under study (e.g., physiological stress—heart rate and cortisol—and aggressive thoughts), and one independent variable, the factor proposed to change the behavior under study (e.g., type of video game). The independent variable is manipulated or varied systematically by the researcher during the experiment (e.g., a child plays with a violent or a nonviolent video game). The dependent variable is expected to change as a result of varying the independent variable, and how it changes is thought to depend on how the independent variable is manipulated (e.g., physiological stress and aggressive thoughts vary in response to the type of video game).

In an experiment, the independent variable is administered to one or more experimental groups, or test groups. The control group is treated just like the experimental group except that it is not exposed to the independent variable. For example, in an experiment investigating whether particular types of music influence mood, the experimental group would experience a change in music (e.g., from “easy listening” to rock), whereas the control group would hear only one type of music (e.g., “easy listening”). Random assignment, whereby each participant has an equal chance of being assigned to the experimental or control group, is essential for ensuring that the groups are as equal as possible in all preexisting characteristics (e.g., age, ethnicity, and biological sex). Random assignment makes it less likely that any observed differences in the outcomes of the experimental and control groups are due to preexisting differences between the groups. After the independent variable is manipulated, if the experimental and control groups differ on the dependent variable, it is concluded that the independent variable caused the change in the dependent variable. That is, a cause-and-effect relationship has been demonstrated.

As another example, consider a study designed to examine whether massage therapy improves outcomes in preterm infants (infants who were born well before their due date) (Abdallah, Badr, & Hawwari, 2013). Infants housed in a neonatal unit were assigned to a massage group (independent variable), who were touched and their arms and legs moved for 10-minute periods once each day, or to a control group, which received no massage. Other than the massage/no-massage periods, the two groups of infants were cared for in the same way. Infants who were massaged scored lower on the measure of infant pain and discomfort (including indicators such as heart rate, oxygen saturation, and facial responses) at discharge (dependent variable). The researchers concluded that massage therapy reduces pain responses in preterm infants.

By experimentally manipulating which infants receive massage therapy, researchers determined that massage can help preterm infants gain weight.

AP Photo / AL GOLDIS

Developmental scientists conduct studies that use both correlational and experimental research. Studying development, however, requires that scientists pay close attention to age and how people change over time, which requires the use of specialized research designs, as described in the following sections.