Before 1970 there was not much recorded information about twins, however there are now larger information sources and larger samples have become available. In Sweden and Finland the central recording of twins raised together and apart also improvements in the interpretation of statistics and technology enabled larger samples to be examined.
Findings from heritability studies have shown that personality traits are associated with genetic influence, however this can vary depending on the trait measured, eg neuroticism shows heritability as 30% but measurements of extraversion/introversion show heritability as 50%.
Measurements could be inaccurate as variables have to be considered eg environmental factors - twins that have been reared apart may have still lived with a family member and could still be brought up in a similar way. The media stories have focused on successful reunions of twins previously separated, twins separated may spend years getting to know each other before they are researched which would be enough time to adapt their personalities. The time separation aspect could mean different things, eg some twins researched had actually spent 11/18 years together. Pederson et al (1988) used samples from Swedish Twin Registry ensuring that twins really did not know each other. His sample was made up of 328 twins of which 99 Monozygotic reared apart and 229 Dizygotic reared apart and 372 twins of which 160 Monozygotic reared together and 212 Dizygotic reared together. He also matched variables ie age, gender & country of birth. From this study the estimate of neuroticism in heritability was 25%.
Twin studies have often used different measures of person, therefore the results may be unreliable unless the same measurement is taken each time. Pederson used the short form of Eysencks personality inventory and used less accurate than measurements used in other research so estimates may be unreliable. Eaves et al (1989) re-analysed all twin studies up to 1976 and found that the estimate of heritability of neuroticism to be 29%. Loehlin & Nichols (1976) studied 800 twins and found heritability of neuroticism to be 53%. Loehlin reanalyzed all the findings from around the world that used behaviour genetic designs to assess heritability of the five factor personality traits and found heritability of neuroticism to be 31%. Extraversion most commonly gave the largest estimate of heritability.
Other research in families involved examining differences in relatedness between other family members ie siblings, half-siblings, cousins, parents and children. There have also been adoption studies.
Research has shown that nature has a proportion of about a third effect on personality, which would infer the larger effect on a personality derives from the environment. There is continual interaction between an organism and its environment from conception throughout life, environments that are both physical and made up with other people. Individual differences psychologists describe environments as ‘dimensions’ ie whether the child has been reared in a controlled or a relaxed environment, the mothers health and well being and the parents relationship.
Within a family each child is different, psychologists, behaviour geneticists and developmental psychologists collaborated to explore how children in same families can differ. (Dunn and Plomin, 1990; Hetherington et al.,1994. Each child experiences the environment differently, they then learn to adapt. Children shape their family environment and behaviour and parents behaviour from an early age, within the same family children will then be treated differently from their siblings. Families also change over time (eg economically, or parents relationships) which could affect a child’s developing personality. MZ siblings may be treated the same because they look the same however within this pairing different levels of extraversion have been found. Some families may encourage the likeness of personality but in others descriptions may vary. Meanings and values to temperaments may encourage or limit the development of a child.
Stern 1985’s study showed how a mother tried to modify her baby’s temperament by tuning (mental responses to an infants mood that provide an experience that the infant can use to regulate its mood) when her child showed affection for her with ‘a bright face and some excited arm flapping’ she responded by just saying ’yes honey’ which fell short of matching his behaviour. Her reason for this was that she felt that if she matched him he would focus his behaviour on her instead of his own. She was trying to ensure that his personality differed to that of his father (who was the passive in their relationship). Stern,however, felt that if her behaviour was not altered her underattunements would create a lower-keyed child which contributed to making the son more like the father anyway. The above is a good example of a mother trying to construct her sons personality.
Personalities can change dependant upon the situation that the person is in. Hartshorne & May discovered that some children would cheat when it was to their advantage to do so, despite having knowledge of what is right and wrong.
The Stanford Prison Experiment (1999) is an example of a social experiment where volunteers undertook new roles. Twenty four were selected after various tests to rule out medical, psychological and other tendencies. They were told that the research was to study the psychological effects on prison life, the experiment took place in a mock prison environment within the university. The experiment was cut short because prisoners became demoralized and showed signs of stress, depression and others lost touch with reality. Guards behaved in authoritarian and aggressive way, some became sadistic. Zimbardo himself became authoritarian and aggressive. The research here showed that social situations have powerful effects on behaviour.
Researchers have used various forms of evidence that seem to indicate that personality is developed through an interaction between genetics and the environment. Differing situations should be accounted for when comparing the results, to date it has been difficult to define what the exact ratio of heritability – environment is, research has shown heritability to be approximately 30%.
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References
Thomas, K The individual differences approach to personality, in Miell, D, Phoenix A and Thomas K (eds), Mapping Psychology 1 Milton Keynes, The Open University.
