410 Part V Biological/Evolutionary Theories
Evidence for the biological basis of personality comes from many differ-
ent sources, including temperament, behavioral genetics, and brain measure
research. First, temperament is the biologically based tendency to behave in
particular ways from very early in life. In one study, for example, Janet DiPietro
and her colleagues (1996) showed that fetal activity and fetal heart rate predict
temperament differences over the first year of life. In particular, a high heart
rate in a 36-week-old fetus foreshadowed less predictable eating and sleeping
habits at 3 and 6 months after birth. A high heart rate also predicted a less
emotional infant at 6 months after birth. The prenatal environment may play an
important role in shaping personality. In fact, the amount of stress the mother
experiences during pregnancy may alter the infant’s own stress response. That
is, infants born to mothers who have experienced an unusual amount of stress
during pregnancy tend to have impaired stress function; higher baseline levels
of stress hormones; and a faster, stronger, and more pronounced physiological
response to stress, all of which persist into childhood (Barbazanges et al., 1996;
Clark & Schneider, 1997).
Secondly, to understand how heredity affects behavior and personality,
psychologists turn to the science of behavioral genetics or the scientific study
of the role of heredity in behavior (Fuller & Thompson, 1960). The extent to
which a characteristic is influenced by genetics is known as heritability, as we
saw in the McCrae and Costa chapter. Researchers use twin-adoption studies
and gene-by-environment studies to study heritability. Twin-adoption studies
research into hereditary influence on twins, both identical and fraternal, who
were raised apart (adopted) and who were raised together. A second technique
in the study of heritability, gene-by-environment interaction research, allows
researchers to assess how genetic differences interact with environment to pro-
duce certain behavior in some people but not in others (Moffitt, Caspi, &
Rutter, 2005; Thapar, Langley, & Asherson, 2007). Instead of using twins,
family members, and adoptees to vary genetic similarity, gene-by-environment
studies directly measure genetic variation in parts of the genome itself and
examine how such variation interacts with different kinds of environments to
produce different behaviors.
Thirdly, biological aspects of personality are assessed using brain imaging
techniques, the two most common forms of which are the electroencephalography
(EEG) and functional magnetic resonance imaging (fMRI). Researchers use elec-
troencephalography (EEG) to record the electrical activity of the brain. The pro-
cedure involves placing electrodes on a person’s scalp. The electrodes, metal disks
attached to wires, are usually mounted in a fabric cap that fits snugly over the
head. Typically, the person is conducting certain tasks while electrical activity is
recorded. EEG is superior to other brain imaging techniques in showing when brain
activity occurs. It is not very accurate at indicating precisely where activity occurs.
Functional MRI (fMRI), does, however, tell us about brain activity. Images from
fMRI tell us where activity in the brain is occurring during particular tasks by
tracking blood oxygen use in brain tissue. In this way, researchers can see which
areas of the brain are using the most oxygen (and presumably are most active)
during certain tasks (Lagopoulos, 2007).