identical on average at the beginning of the experiment. For example, people
who watch relatively little television might be more intelligent or come from ahigher socioeconomic level. We can be fairly certain that they have more time
for activities other than television, such as reading or interacting with friends.
The two kinds of participants also might differ in terms of self-esteem, diet,
and, most notably, their level of aggression prior to participating in the exper-
iment. Thus any differences we find between the two groups could be caused
by any of these differences, and not necessarily by the number of violent TVshows each group watches.
Because it is difficult to determine cau se-and-effect relationships with non-
manipulated independent variables, researchers generally prefer to manipulatevariables. However, doing so is not always possible. Sometimes manipulating
the variable is too expensive, too difficult, or unethical. This is a particular prob-
lem in personality research because many of the variables researchers want tostudy simply cannot be manipulated. Returning to the violent television exam-
ple, it would be next to impossible to tell some participants, “You watch a lot
of violent television during the next few years, ”and tell others, “You watch no
violent television until I tell you it ’s okay. ”Instead, if we want to know about
the long-term effects of exposure to violent TV, we have to accept the partici-
pants as they are, understanding that many group differences exist at the
outset of the study. Sometimes investigators try to control some of these known
differences, such as by comparing the education levels of the two groups.
However, researchers can never be sure that they have controlled all relevant
variables.
This is not to say that research with nonm anipulated independent variables
is useless. On the contrary, personality psychologists often find that relying onnonmanipulated variables is the only w ay to examine a topic of interest. How
else can we study differences between introv erts and extraverts or differences be-
tween men and women? A recent survey of academic journals found that thevast majority of personality research relies on nonmanipulated independent vari-
ables (Revelle & Oehlberg, 2008). Nonetheless, investigators who conduct this
research must remain cautious when making statements about cause and effect.
Prediction Versus Hindsight
Which person is more impressive: the one who can explain after a basketball
game why the winning team was victorious, or the one who accurately
tells you before the game which team will win and why? Most of us are
more impressed with the second person. After all, anyone can come up with
an explanation after the facts are in. But people who really understand
the game can make reasonable guesses about what will happen when two
teams meet.
In a similar manner, if a scientist has a legitimate theory, we can expect him
or her to make reasonably accurate predictions of what will happen in a studybefore the data are in. Remember, the purpose of research is to provide supportfor a hypothesis. Researchers generate a theory, make a hypothesis, and collect
data that either support or do not support t he hypothesis. Suppose a researcher
examines the relationship between sel f-esteem and helping behavior, but the26 CHAPTER 2 Personality Research Methods