An interesting research method used by child psychologists to determine which traits and behaviours are inherited is by comparing monozygotic twins and dizygotic twins. This provides a way of isolating genetic influence. The rationale is that since monozygotic (identical) twins are born from the same zygote (an ovum that has been fertilised by a sperm cell), they will have an identical genetic makeup. Dizygotic twins are born from two different zygotes; thus their genetic makeup differs as much as any two siblings genetic makeup would.

For example, in a study to determine if intelligence is genetically determined, the researcher will want to learn if the intellectual capacity of identical twins is more similar, or more closely correlated than that of dizygotic twins. If it is (and this has actually been discovered to be correct), then the evidence indicates that intelligence is largely genetically determined.

Experiments with identical twins have testified to a high level of genetic influence on personality development. Comparisons were made between a pair of identical twins reared together, and a pair reared apart. One would expect the personality of those reared in the same family to be more similar because of the same environmental influences. The opposite, however, proved to be true. Those reared apart were more similar in temperament. A possible interpretation of these results is that identical twins reared together tend to feel pressured to develop distinctive individual characteristics. This research emphasises that genetic influence on psychological development is by no means negligible.

Based on her research with separated twins, Susan Farber (Wade and Tavris, 1987) identifies the following aspects of personality as likely to be inherited:

  • 1-Nervous habits and mannerisms
  • 2-Mood or general outlook (optimistic, gloomy, moody)  fearfulness and anxiety.

Other researchers have identified certain other characteristics that seem to be present in infants from their first weeks into adulthood, suggesting that these characteristics are genetically determined. They include:

  • 1-Cheerfulness and adaptability
  • 2-Fussiness and fearfulness
  • 3-passivity
  • 4-Emotionality
  • 5-Sociability
  • 6-Activity
  • 7-Introversion or extroversion.

Whether or not these tendencies will develop into personality traits in the adult will depend on environmental influences. Calm easy-going parents might have a calming effect on an irritable child. A passive child can learn to be more outgoing and responsive, especially when around older siblings who are.