INTRODUCTION: GENETICS, BIOLOGICAL CAUSES OF BEHAVIOR, AND CONSTITUTIONAL PSYCHOLOGY

Twin studies have been used to investigate genetic (biological) influences on personality. Identical (monozygotic-MZ) twins that share 100% of their genes in common are compared with Fraternal (dizygotic-DZ) twins that share 50% of their genes in common.

One member of the twin pair is arbitrarily called the Index case and the other twin is called the Proband. Generally, all the Index cases share a certain characteristic (e.g., color blindness, or left- handedness, or schizophrenia), and the researchers determine what percentage of the Probands have the same characteristic (called a concordance rate).

If the concordance rate for MZ twins is significantly higher than the rate for DZ twins, then there is evidence supporting a genetic contribution to the characteristic that is measured. However, such significant data does not mean that the characteristic is totally genetically caused.

Research has shown that it is likely that there are genetic influences on three TEMPERMENTS:

1. Sociability

2. Emotionality, and

3. Activity level.

But remember that there are also likely to be environmental influences on these three temperaments.

There is also evidence for heritability of the Big Five personality dimensions.

Also, if identical twins that are raised apart show greater similarities in personality or behavior than unrelated people that are raised apart, this would also show support for genetic influences on personality.

ALLELES have been identified on the human genome that are related to the use of important neurotransmitters, such as DOPAMINE and SERATONIN, whose actions will be considered later in the course.

CONSTITUTIONAL PSYCHOLOGY is a body of theory and research that assumes that characteristics of a person’s physique (body structure) are related to a person’s personality and behavior.

The history of constitutional psychology includes Phrenology (Gall & Spurzheim) that held that bumps on one’s skull influenced behavior. This theory has been disproved.

Kretchmer was a psychiatrist, who identified three types of physiques:

Asthenic - Tall, thin people

Athletic - Muscular people

Pyknic - short, fat people, and a 4th category

Dysplastic - unusual body types.

Kretchmer found that individuals with schizophrenia were more likely to have ASTHENIC AND ATHLETIC body types, while people with manic-depression were more likely to have PYKNIC body types. However, Kretchmer’s data did not “prove” that Physique caused abnormal behaviors because (A) Kretchmer’s data were only correlational, and (B) there were other alternative interpretations of the data. Alternative interpretations are also known as ARTIFACTS or CONFOUNDS.

WILLIAM SHELDON is the most well-known constitutional psychologist who did better research than Kretchmer. Sheldon identified 3 components of physique:

Endomorphy -- short and fat

Mesomorphy -- muscular

Ectomorphy -- tall and thin

Each person’s physique could be rated on these three components using a SOMATOTYPE rating, which is a 3 digit rating with each digit ranging from 1 (low on the component) to 7 (high on the component. 4 is average.

For example, a somatotype of E. Johnson a 6 ft. 11 in professional basketball player would be

1-5-6. The first digit is always Endomorphy, the 2nd is always Mesomorphy, the 3rd is always Ectomorphy.

Sheldon also identified 3 components of personality:

Vicertonia (very sociable and outgoing), Somatotonia (energetic and aggressive), and Cerebrotonia (shy and inhibited).

In research, Sheldon found the following correlations between Physique and Personality.

Vicerotonia Somatotonia Cerebrotonia

Endomorphy +.79 -.29 -.32

Mesomorphy -.23 +.82 -.58

Ectomorphy -.40 -.53 +.83

The correlations in the diagonals are significant, indicating that Endomorphs are more likely to have personalities strong in Vicerotonia, Mesomorphs are more likely to have personalities strong in Somatotonia, and Ectomorphs are more likely to have personalities strong in Cerebrotonia.

There are at least four possible explanations for this correlational data.

1. Direct biological influence

2. Direct environmental influence

3. Indirect biological influence (interaction), biological effects on behavior are indirect when the biological variable moderates (changes) the potential impact of the environment on behavior by (A) restricting or facilitating the range of environmental experiences one may have (e.g., beauty attracts others, deformity repels others, blindness deprives one of visual feedback from the environment) or (B) limiting the amount of impact of the environment (e.g., the genetic defect of Downs Syndrome - Mongolism- limits the impact of an intellectually stimulating environment on IQ).

4. Societies stereotypes. (People behave in ways that are expected of them)