in middle school students and found that children with higher aerobic capacity scored higher on achievement tests than did children with poorer aerobic capacity (Bass, Brown, Laurson, & Coleman, 2013). Note that this correlation does not tell us why aerobic capacity was associated with academic achievement. Correlational research cannot answer this question because it simply describes relationships that exist among variables; it does not enable us to reach conclusions about the causes of those relationships. It is likely that other variables influence both a child’s aerobic ability and achievement (e.g., health), but correlation does not enable us to determine the causes for behavior—for that we need an experiment.
Researchers experimentally manipulate which children play with violent video games to determine their effect on behavior.
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Experimental Research
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.
Brain and Biological Influences on Development
Methods of Studying the Brain
Modern brain imaging techniques enable us to measure brain activity as individuals think and solve problems.
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What parts of the brain are active when we solve problems or feel emotions? How does the brain change with development? Until recently, the brain was a mystery. Over the past hundred years, researchers have devised several ways of studying brain activity that have increased our understanding of how the brain functions and how it develops.
The earliest instrument created to measure brain activity was the electroencephalogram, first used with humans in the 1920s (Collura, 1993). Electroencephalography (EEG) measures electrical activity patterns produced by the brain via electrodes placed on the scalp. Researchers study fluctuations in activity that occur when participants are presented with stimuli or when they sleep. EEG recordings measure electrical activity in the brain, but they do not provide information about the location of activity.
Not until the invention of positron emission tomography (PET) in the early 1950s did researchers obtain the first glimpse of the inner workings of the brain (Portnow, Vaillancourt, & Okun, 2013). Researchers inject a small dose of radioactive material into the participant’s bloodstream and detected by the PET scan. The radioactive material enables researchers to monitor the flow of blood. Blood flows more readily to active areas of the brain, and the resulting images can illustrate what parts of the brain are active as participants view stimuli and solve problems. Developed in 1971, computerized tomography, known as the CT scan, produces X-ray images of brain structures (Cierniak, 2011). A movable X-ray unit rotates around a person’s head as it records images of the brain (Herman, 2009). The images are then combined to make a three-dimensional picture of the person’s brain, providing images of bone, brain vasculature, and tissue. CT scans can provide researchers with information about the density of brain structures to illustrate, for example, how the thickness of the cortex changes with development.
Functional magnetic resonance imaging (fMRI) measures brain activity by monitoring changes in blood flow in the brain (Bandettini, 2012). Developed in the 1990s, MRI machines house a powerful magnet that uses radio waves and to measure blood oxygen level. Active areas of the brain require more oxygen-rich blood. Like PET scans, fMRI enables researchers to determine what parts of the brain are active as individuals complete cognitive tasks. However, fMRI images are much more detailed than PET scans. An important advantage of fMRI over PET scans is that it does not rely on radioactive molecules, which can only be administered a few times before becoming unsafe.
Another imaging process, called diffusion tensor imaging (DTI), uses an MRI machine to track how water molecules move in and around the fibers connecting different parts of the brain (Soares, Marques, Alves, & Sousa, 2013). DTI gauges the thickness and density of the brain’s connections, permitting researchers to measure the brain’s white matter and determine changes that occur with development and with age-related illnesses, such as Alzheimer’s disease.
What Do You Think?
If you were going to study the brain, which measure would you choose and why? What type of information would you obtain from your chosen measure? Identify a research question that your measure might help you answer.
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. The researchers 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. They 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 gender). 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
