relatively few. You then measure the participants ’level of aggression in a num-
ber of situations. Consistent with the hypothesis, you find people who watch a
lot of violent television are more aggressive than those who watch relatively lit-
tle violent TV. You might be tempted to conclude that watching violent televi-
sion programs causes people to be more aggressive. However, based on this
study alone, your conclusion must be tempered. For example, it ’s possible that
these people watch violent TV shows precisely because they are aggressive. Per-
haps they are more entertained by programs that include shootings, stabbings,and other violent acts. Thus, although the findings are consistent with the hy-
pothesis, statements about cause and effect must be qualified.
This example illustrates the fundamental difference between research
using manipulated independent variables and research using nonmanipulatedindependent variables. An investigator who uses a manipulated independent
variable begins with a large number of participants and randomly assigns
them to experimental groups. That is, each person has an equally likely
chance of being assigned to Condition A as to Condition B (or C, or D, and
so on). Investigators know all participants are not exactly alike at the begin-ning of the study. Some are naturally more aggressive than others, some
more anxious, some more intelligent. Each has different life experiences that
might affect what he or she does in the study. However, by using a large
number of participants and randomly assigning them to conditions, research-
ers assume that all of these differences will be evened out. Thus, although
within any given condition there are people who are typically high or low in
aggressiveness, each condition should have the same average level of aggres-
siveness at the beginning of the experiment.
The researcher then introduces the independent variable. For example, one
group might be shown 30 minutes of violent television programming, another
group might watch a baseball game, and still another group might sit quietly
and watch no television. Because we assume participants in each condition arenearly identical on average at the start of the study, any differences among the
groups after watching the program can be attributed to the independent vari-
able. That is, if participants who watched the violent TV shows are more ag-
gressive than those who watched the nonviolent shows or those who watched
no TV, we can conclude with reasonable confidence that watching the violent
TV shows caused the participants to act more aggressively.
This procedure contrasts with one that uses nonmanipulated variables.
Anonmanipulated independent variable (sometimes referred to as a subject
variable ) exists without the researcher ’s intervention. For example, research-
ers might divide people into high self-esteem and low self-esteem groups, or
into first-born, middle-born, or last-born categories. In these cases, the inves-
tigator does not randomly assign participants to a condition. Returning to theearlier example, the researcher who compared frequent and infrequent televi-
sion viewers did not manipulate participants into those two categories.
Instead, the participants had already determined which of the groups theybelonged to without any action on the researcher ’s part.
The difficulty with this and other nonmanipulated independent variables
is that the researcher cannot assume the people in the two groups are nearly“Personality is so
complex a thing that
every legitimatemethod must be em-
ployed in its study. ”
Gordon AllportThe Hypothesis-Testing Approach 25