2 verities of animal studies

  • Selective breeding studies: manipulate heredity which keeping environment constant  Mice: breed 2 mice high in traits and low in traits.
  • Raise the animal over several generations. Raise animals with high trait A and those high trait B. measure degree to which they vary = amount due to heredity.

In breeding = manipulate environment while genetics remain constant. Breed offspring back to parents. About 20 generations. Get identical = any changes are due to environment.

Condition 1: hereditary difference and environment constant.

  • Difference found for height in mice = .90. (reared apart) Example where heredity is affected by environment

Fraternal twins raised in the same household or identical twins in the same household.

 Get as much similarity in environment, but fraternal twins share an average of 50% of genes in common whereas identical twins is 100% of their genes in common.

  • If there is a different between identical twins and fraternal then heredity would be part of that.
  • Fraternal twins in height: .45
  • And identical reared APART: .90

      You know that genetics are involved.

Siblings are reared apart from birth. Environments are not similar:

  • The correlation between twins apart is the same as twins reared together.
  • No effect of shared environment: being raised in the same household.

Fraction that determines the role of genetics  heritability

Def’n: the proportion of phenotypic variation is due to genotypic variation.

  • How much of the difference between individuals is based on heredity.
  • How do we measure this? The correlation between identical twins reared apart on some traits.  Assume they have no been adopted into families that are like their biological ones  Any attribution of a trait is due to heritability.

Randomly match between 2 pairs  assume there is a degree of association is heredity.

Environments of identical twins are not always identical.

Correlation of identical twins reared together, and fraternal twins reared together  heritably can be calculated in a simple equation.

Fraternal twins: half the genetic influences, plus all the influences of shared environment.

Identical twins: genetic influences and influences of shared environment

What is the heritability of height based on? Subtract fraternal twins by identical twins and then double it.

Uncorrelated environments: IQ. Heritability for IQ.

Identical twins reared apart: correlation is .72 which is quite high.

Identical twins reared together: .86

Fraternal twins reared together: .60

  • Subtract .60 from .86 and then double it  .52. a more accurate assessment of HERITABILITY OF IQ.
  • Half heredity, half environment

Compare adopted children with their biological and adopted parents.

  • Half siblings with full siblings.
  • Can see if they are raised together or apart.

More advanced statistical methods which give them more advanced measures which show the end of genetic variance.

Additive variance: computable linear relationships Nonadditive: no interaction of any sort.

Data of the traits studied by psychologists

Widest studied trait is IQ: convienent, available, encompasses a wide range of things.

  • As people get older, heritability of IQ increases, in middle childhood and adolescence.
  • The older the group, the more heritability will show.
  • Parents are less involved with their kids as they are older, so the biological influences take over.
  • As adopted children get older, their IQ and intelligence will become more like their parents. As children got older, they wanted to meet their parents.
  • Twins raised apart, reunited, you can see the nuances of behaviour that they show.

Many studies on cognitive inheritability: .30 – .50 for verbal comprehension, fluency, spatial visualization, verbal reasoning etc.

Reading ability: .30 upward

Personality – most studies focused on 3 traits. Eysneck’s theory which was that personality had 3 components

  • Emotionality – tendency to have arousing emotions
  • Extraversion
  • Activity level

.30 or higher using the Kreuger method (?). Minnesota project.

To get estimates of .50 and .75, it includes optimism (sensibility). This is the most heritable personality trait.  .82 Others:

  • Leadership
  • Social closeness
  • Alienation
  • Aggression
  • Self-control
  • Altruism/empathy
  • Nurturance

(Moderate number of inheritability).

       Political conservatibility  .30 and .50.

–       Does not mean there is a gene for this, but there may be heritable traits that cause this.

Heritability for the 2 major forms of psychosis

  • Schizophrenia (manic depressive)
  • Bi polar disorder

Shyness and aggression do have reasonably high heritability traits

 Also a high hereditability for criminal acts, because of testosterone level – genetically determined.

  • This makes you more aggressive.

High T and raised in a low-socio economic environment, you will have a higher aggression. If you are raised with high T in a middle class society then you are likely to have less criminal activity.

Estimates vary between 50-60 % for most traits, but a lot of study shows nill effects of heritability in shared environments. The correlations for siblings apart are the same as those reared together.

  • For unrelated siblings reared together, tend to be nill. Similarities of the environment do not account in themselves for personality similarities.
  • Antithetical to personality development theories.
  • Siblings reared apart have the same correlations as siblings reared together.
  • Can have the same environment but different treatment in the environment.
  • Know key percentages, ex. Heritability is 50. Not every individual personality trait.

Vp (variance of pheotype)/Vg(variance of genetics) this is simple heritability or H squared.

The minus and doubling is a more accurate measurement of heritability than the simple measure. Additative hereditability: multiple genes at the same locus that can contribute to the same trait.