Developmental psychology.pdf

investigating Behavior 47
Figure 2.11
Identical Twins. The product of a
single fertilized egg, it is sometimes
difficult to determine when twins are
identical. Studies of blood type, skin
grafts, and the mother's placenta are
sometimes necessary.
except for this variable's presence. Since unwanted influences should occur equally in
both conditions, any difference in the subject's response must be due to the influence
of the experimental or independent variable.*
This overall plan for different conditions and treatments is called the design
of the experiment. It involves the choice of variables, subjects, and apparatus, but no
issue is more important than control. The reason is that extraneous variables can influence
the outcome of the research, giving the experiment an uninterpretable result.
The purpose of controls is to provide a clearer indication of the effect of the independent
variable on the dependent variable.
Own Controls One procedure for choosing subjects is called the method of own
controls because all subjects serve in both conditions, experimental and control. Each
subject in the experimental condition is compared with himself in the control condition.
In this way the two conditions should be equal, except for the key factor, the independent
variable.
Maury, under the experimental condition, would be exposed to a heated iron
or to the bell or to some other stimulus serving as the independent variable. Under the
control condition he would not be stimulated in this way, but the next morning he
would always be asked to report his dreams. After several trials under each condition,
using many additional subjects if possible, the contents of the dreams under the two
conditions would be compared.
'Can dogs construct
recognizable images from a
television screen? For the first
group, place some hungry dogs—
who have not eaten for eight
hours—in front of a TV that is
broadcasting an ad showing
various types of dog food. For the
second group, place dogs of the
same age and breed who have just
eaten in front of the same TV. ...
Two Groups of Subjects Sometimes the same subjects cannot serve in both
conditions. This limitation is especially prevalent when the independent variable extends
over a long period of time, as in a program of therapy. If the therapy lasts for two
years, the experimental subjects will be two years older and wiser, apart from the
therapy, when they begin the control condition. Several methods exist for dealing with
this problem, one of which involves two sets of subjects.
" When control of inherited factors is desired, identical twins are used; one twin
is placed in the experimental group and the other in the control group. This method
of co-twin control is effective because identical twins have the same heredity and also
because maturation is largely influenced by hereditary factors. A similar technique
with animals is the split-litter technique, in which a litter of six pups is divided into
three pups for 4 the experimental group and three for the control group, and these assignments
are randomly determined (Figure 2.11).