Learning Objectives

  • Describe the experimental process and its importance to abnormal psychology

Causality: Conducting Experiments and Using the Data

As you have learned, the only way to establish that there is a cause-and-effect relationship between two variables is to conduct a scientific experiment. Experiment has a different meaning in the scientific context than in everyday life. In everyday conversation, we often use it to describe trying something for the first time, such as experimenting with a new hair style or a new food. However, in the scientific context, an experiment has precise requirements for design and implementation.

Experimental Hypothesis

In order to conduct an experiment, a researcher must have a specific hypothesis to be tested. As you have learned, hypotheses can be formulated either through direct observation of the real world or after careful review of previous research. For example, if you think that children should not be allowed to watch violent programming on television because doing so would cause them to behave more violently, then you have basically formulated a hypothesis—namely, that watching violent television programs causes children to behave more violently. How might you have arrived at this particular hypothesis? You may have younger relatives who watch cartoons featuring characters using martial arts to save the world from evildoers with an impressive array of punching, kicking, and defensive postures. You notice that after watching these programs for a while, your young relatives mimic the fighting behavior of the characters portrayed in the cartoon (Figure 1).

A photograph shows a child pointing a toy gun.

Figure 1. Seeing behavior like this right after a child watches violent television programming might lead you to hypothesize that viewing violent television programming leads to an increase in the display of violent behaviors (credit: Emran Kassim).

These sorts of personal observations are what often lead us to formulate a specific hypothesis, but we cannot use limited personal observations and anecdotal evidence to rigorously test our hypothesis. Instead, to find out if real-world data supports our hypothesis, we have to conduct an experiment.

Designing an Experiment

The most basic experimental design involves two groups: the experimental group and the control group. The two groups are designed to be the same except for one difference— experimental manipulation. The experimental group gets the experimental manipulation—that is, the treatment or variable being tested (in this case, violent TV images)—and the control group does not. Since experimental manipulation is the only difference between the experimental and control groups, we can be sure that any differences between the two are due to experimental manipulation rather than chance.

In our example of how violent television programming might affect violent behavior in children, we have the experimental group view violent television programming for a specified time and then measure their violent behavior. We measure the violent behavior in our control group after they watch nonviolent television programming for the same amount of time. It is important for the control group to be treated similarly to the experimental group, with the exception that the control group does not receive the experimental manipulation. Therefore, we have the control group watch non-violent television programming for the same amount of time as the experimental group.

We also need to precisely define, or operationalize, what is considered violent and nonviolent. An operational definition is a description of how we will measure our variables, and it is important in allowing others understand exactly how and what a researcher measures in a particular experiment. In operationalizing violent behavior, we might choose to count only physical acts like kicking or punching as instances of this behavior, or we also may choose to include angry verbal exchanges. Whatever we determine, it is important that we operationalize violent behavior in such a way that anyone who hears about our study for the first time knows exactly what we mean by violence. This aids peoples’ ability to interpret our data as well as their capacity to repeat our experiment should they choose to do so.

Once we have operationalized what is considered violent television programming and what is considered violent behavior from our experiment participants, we need to establish how we will run our experiment. In this case, we might have participants watch a 30-minute television program (either violent or nonviolent, depending on their group membership) before sending them out to a playground for an hour where their behavior is observed and the number and type of violent acts is recorded.

Ideally, the people who observe and record the children’s behavior are unaware of who was assigned to the experimental or control group, in order to control experimenter bias. Experimenter bias refers to the possibility that a researcher’s expectations might skew the results of the study. Remember, conducting an experiment requires a lot of planning, and the people involved in the research project have a vested interest in supporting their hypotheses. If the observers knew which child was in which group, it might influence how much attention they paid to each child’s behavior as well as how they interpreted that behavior. By being blind to which child is in which group, we protect against those biases. This situation is a single-blind study, meaning that one of the groups (participants) are unaware as to which group they are in (experiment or control group) while the researcher who developed the experiment knows which participants are in each group.

A photograph shows three glass bottles of pills labeled as placebos.

Figure 2. Providing the control group with a placebo treatment protects against bias caused by expectancy. (credit: Elaine and Arthur Shapiro)

In a double-blind study, both the researchers and the participants are blind to group assignments. Why would a researcher want to run a study where no one knows who is in which group? Because by doing so, we can control for both experimenter and participant expectations. If you are familiar with the phrase placebo effect, you already have some idea as to why this is an important consideration. The placebo effect occurs when people’s expectations or beliefs influence or determine their experience in a given situation. In other words, simply expecting something to happen can actually make it happen.

The placebo effect is commonly described in terms of testing the effectiveness of a new medication. In a placebo condition, the placebo has inert ingredients.

Imagine that you work in a pharmaceutical company, and you think you have a new drug that is effective in treating depression. To demonstrate that your medication is effective, you run an experiment with two groups: The experimental group receives the medication, and the control group does not. But you don’t want participants to know whether they received the drug or not.

Why is that? Imagine that you are a participant in this study, and you have just taken a pill that you think will improve your mood. Because you expect the pill to have an effect, you might feel better simply because you took the pill and not because of any drug actually contained in the pill—this is the placebo effect.

To make sure that any effects on mood are due to the drug and not due to expectations, the control group receives a placebo (in this case a sugar pill). Now everyone gets a pill, and once again neither the researcher nor the experimental participants know who got the drug and who got the sugar pill. Any differences in mood between the experimental and control groups can now be attributed to the drug itself rather than to experimenter bias or participant expectations (Figure 2).

When it comes to abnormal psychology, it is common practice in studies evaluating therapy effectiveness to have a placebo condition. In a placebo condition, participants receive a treatment similar to the experimental treatment, but lacking the key feature of the treatment of interest. If the study is evaluating effectiveness of therapy, psychologists must design the placebo in a way that is very similar, but not the same as the actual therapy. Ideally, we would want the placebo participants to receive treatments in the same modality, or form in which the clinician offers psychotherapy, as the experimental group participants, including frequency and duration.  

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Independent and Dependent Variables

In a research experiment, we strive to study whether changes in one thing cause changes in another. To achieve this, we must pay attention to two important variables, or things that can be changed, in any experimental study: the independent variable and the dependent variable. An independent variable is manipulated or controlled by the experimenter. In a well-designed experimental study, the independent variable is the only important difference between the experimental and control groups. A dependent variable is what the researcher measures to see how much effect the independent variable had. In our example, the dependent variable is the number of violent acts displayed by the experimental participants. Examples of independent variables in experimental research in abnormal psychology include different types of drug treatments for psychological treatments, treatment factors such as brief versus long-term treatment or inpatient versus outpatient treatment, and experimental manipulations such as consuming alcoholic versus non-alcoholic beverages. Examples of dependent variables include behavioral variables such as measures of adjustment, activity levels, eating behavior, and smoking behavior; physiological variables such as measures of physiological responses such as heart rate, blood pressure, and brain wave activity; and self-report variables such as measures of anxiety, mood, or marital or life satisfaction.

We expect that the dependent variable will change as a function of the independent variable. In other words, the dependent variable depends on the independent variable. A good way to think about the relationship between the independent and dependent variables is with this question: what effect does the independent variable have on the dependent variable?

Issues to Consider

In research on the causes of abnormal behavior, it may be difficult to design a true experimental study. Many of the variables that psychologists are interested in are precisely ones that the researcher cannot control. One very important question is, can the variable be considered  truly “independent”? Take the example of depression that we illustrated in our discussion about correctional research. Depression in those examples can never be an independent variable because the researcher cannot manipulate it. Another example is that we cannot randomly assign people to groups based on their gender. Quasi-experiments have independent variables that already exist such as age, gender, and eye color. Experimenters are also limited by ethical constraints. For instance, you would not be able to conduct an experiment designed to determine if experiencing abuse as a child leads to pedophilic disorder among adults. To conduct such an experiment, you would need to randomly assign some experimental participants to a group that receives abuse, and that experiment would be unethical. 

A study that investigates differences among groups from a target population without random assignment are known as quasi-experimental. That is to say that this type of study specifically lacks the element of random assignment to treatment or control groups. Random assignment is critical for sound experimental design and means that all participants have an equal chance of being assigned to either group. However, in a quasi-experimental design, assignment to a given treatment condition is based on something other than random assignment. Depending on the type of quasi-experimental design, the researcher might have control over the assignment to the treatment condition but use some criteria other than random assignment (e.g., a cutoff score) to determine which participants receive the treatment, or the researcher may have no control over the treatment condition assignment and the criteria used for assignment may be unknown.

On their own, quasi-experimental designs do not allow one to make definitive causal inferences; however, they provide necessary and valuable information that cannot be obtained by experimental methods alone.

Most “true” experimental studies in abnormal psychology test not the causes of abnormal behavior, but the effectiveness of particular treatments where it is possible to design randomly assigned control and experimental groups. Researchers use traditional, experimental design or randomized, controlled trials to evaluate a treatment’s effectiveness by comparing the groups on dependent variables such as the effectiveness of a new treatment, as discussed earlier in the placebo condition. There may be more than one experimental group, depending on the nature of the study.

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Research Design Review

Learn more about research design and methods in the following interactive.


cause-and-effect relationship: changes in one variable cause the changes in the other variable; can be determined only through an experimental research design

confirmation bias: tendency to ignore evidence that disproves ideas or beliefs

control group: serves as a basis for comparison and controls for chance factors that might influence the results of the study—by holding such factors constant across groups so that the experimental manipulation is the only difference between groups

correlation: relationship between two or more variables; when two variables are correlated, one variable changes as the other does

dependent variable: variable that the researcher measures to see how much effect the independent variable had

double-blind study: experiment in which both the researchers and the participants are blind to group assignments

experimental group: group designed to answer the research question; experimental manipulation is the only difference between the experimental and control groups, so any differences between the two are due to experimental manipulation rather than chance

experimenter bias: researcher expectations skew the results of the study

independent variable: variable that is influenced or controlled by the experimenter; in a sound experimental study, the independent variable is the only important difference between the experimental and control group

operational definition: description of what actions and operations will be used to measure the dependent variables and manipulate the independent variables

placebo condition: participants receive a treatment similar to the experimental treatment, but lacking the key feature of the treatment of interest

placebo effect: people’s expectations or beliefs influencing or determining their experience in a given situation

quasi-experimental: a study that investigates differences among groups from a target population without random assignment

random assignment: method of experimental group assignment in which all participants have an equal chance of being assigned to either group

single-blind study: experiment in which the researcher knows which participants are in the experimental group and which are in the control group