+ The nature/nurture question and how psychologist go about addressing it
- Why am I the way I am? To ask this question presumes that you could be some way other than the way you are, and this presumption stems from the obvious fact that people are different.
+ Why, then, are people different?
+ physical differences
- body size
- musculature
- eye color
- sex
+ physiological differences
- arousal level
- blood pressure
- metabolic rate
- sleep/wake cycles
- alcohol susceptibility
+ psychological differences
- moodiness
- intelligence
- musical aptitude
- aggressiveness (sex related)
- sociability
+ Historical perspective
- Charles Darwin/Gregor Mendel: genetics of individual differences
- Selective breeding studies - animal breeders have known for centuries that traits can be inbred over generations, by breeding two mates who both exhibit the desired characteristic
+ Behavioral genetics: study of genetic and environmental factors that create cognitive, emotional and behavioral differences among individuals
- heredity - genetic transmission of differences from parent to offspring
- group differences are typically very small whereas individual differences can be huge.
- the causes of differences between groups may be different than causes of differences among individuals
+ everyone believes that the environment can make people different
- behaviorism, the dominant theme in psychology, focuses on learning and experience
+ everyday living underscores that experience changes people
- the more you study, the better your grades
- the longer your practice, the better you perform
- the greater your stress level, the more likely you are to develop health problems
- we want to believe that the environment is of paramount importance, for we can change an individual’s environment but we cannot modify that individual’s genetic make-up (at least not yet)
+ Genes and their actions
- The nucleus of each cell in our bodies (except the sex cells) contains 23 pairs of chromosomes, each composed of helix-shaped double coiled DNA; aggregates of DNA comprise our genes
- Total of 3.5 x 109 base pairs in one nucleus (80 million/chomosome); we have approximately 105 genes.
- Our DNA differs from that of chimpanzees by only 2 - 3%.
- 99.9% of our DNA is common to all humans; genetically-based individual differences are attributable to the remaining 0.1%.
- DNA represents instructions for creation of protein, and it is now possible to isolate DNA from humans, attach it to a virus and synthesize the protein specified by that DNA
- DNA also replicates and is transmitted to offspring (either sexually or asexually)
- Most physical and behavioral characteristics (phenotypes) are not determined by single genes but by hosts of genes (genotypes) acting in concert (the property termed polygeny), which explains why discrete entities (genes) can give rise to normally distributed characteristics
+ Genes code for amino acid sequences, not behavior
- strictly speaking, there is no such thing as a gene for alcoholism: there are genes that influence rate of alcohol uptake from the blood and regulate the number of receptors on brain cells responsive to active ingredients in alcohol
- strictly speaking, there is no such thing as a gene for depression: there are genes that regulate synthesis of neurochemicals and the receptors for those neurochemicals
+ How can genetic influences on behavior be detected?
+ Four key propositions
- genetic differences among individuals can produce phenotypic (observed) differences
- nongenetic differences among individuals can produce phenotypic differences
- to the extent that genetic differences contribute to phenotypic differences, phenotypic similarity among relatives should vary according to their genetic similarity (which can be estimated)
- to the extent that shared environmental factors influence a particular trait, phenotypic similarity will also exist among relatives who have lived together
+ Polymorphic relations among individuals
- Monozygotic twins - two individuals who develop from a single fertilized egg; genetic similarity is 100% (clones)
- Dizygotic twins - two individuals who develop from a pair of fertilized eggs; genetic similarity is 50%
- First-degree siblings - two (or more) individuals with common mother and father; genetic similarity is 50%
- Second-degree siblings - individuals with one parent in common; genetic similarity is 25%
+ Correlations for trait entirely genetically determined (a condition that does not exist in psychology)
+ MZ twins:
- 1.0
+ DZ twins and siblings:
- 0.50
+ 2nd degree sibs:
- 0.25
+ Correlations for trait in which heredity accounts for 50% of the variation among individuals
+ MZ:
- 0.50
+ DZ:
- 0.25
+ 2nd degree:
- 0.125
- Basic problem with family studies: individuals share genetic makeup and environment. How can these be untangled?
+ Twin studies
- Approximately 1 in 85 births are twins, equally distributed between MZ and DZ (twinning itself has genetic component).
+ Compare MZ and DZ (same gender, to match MZ case) twins reared together
- If trait has no heritability, correlation for MZ and for DZ should be equivalent.
- Possible problem: MZ twins may experience a more similar environment than do DZ twins (people treat MZ twins differently than DZ twins), and this could inflate the correlation for MZ relative to DZ
- Can’t draw any conclusions from single comparison of one set of MZ twins and one set of DZ twins (that’s why flashy news stories about twins are just that - flashy news stories); to compute correlations requires a large sample
+ Adoption studies
+ Genetically similar individuals raised apart; similarities are attributable to heredity.
+ MZ reared apart
- r = .90 for height
+ Siblings reared apart
- upper limit on correlation will be r = 0.50; this would indicate that the trait in question had a heritability index of 100% (i.e., it is determined entirely by heredity)
+ Genetically unrelated individuals reared together (adopted siblings)
+ For a trait with high heritability, the correlation among such individuals should be close to zero; if a trait shows high correlation among unrelated individuals living together, this constitutes evidence for a strong shared environmental component.
- very low correlation for personality
- very low correlation for adult IQ
- very low correlation for risk for psychopathy
+ Problem with adoption studies:
- adopted apart siblings are sometimes placed in homes that are not randomly selected (i.e., there is correlation of "environment")
- adopted children are sometimes placed with families that resemble the biological family.
+ Characteristics that show significant degree of heritability
+ IQ - strong heritability ( 50%), but this varies with age (being highest in adulthood)
- r for MZ vs DZ = .86 vs .60
- r for MZ reared apart = .72
- School achievement .50
- Reading disability: high
- Incidence of Alzheimer’s disease: relatively high
- Incidence of schizophrenia: moderately high
- Incidence of alcoholism: moderately high
+ Closing thoughts
- Behavioral genetics shows that environment always contributes to individual differences: MZ twins are never identical psychologically
- "Environment" covers a wide range of factors (starting with in utero and including nutrition, trauma, learning opportunities, social interactions)
- Relative contribution of genetics can vary throughout one’s lifetime
+ Distinction between "shared environment" vs "non-shared environment"
- the former accounts for very little variance (except for religious and political beliefs)
+ Why are children reared in the same family often so different from one another? Perhaps there are very few genuine "shared environmental factors."
- Socio-economic status may change, meaning that different children in the same family will experience the effects of this factor differently
- Parents tend to treat first children differently than they treat subsequent children (birth order effect)
- Birth order affects the way siblings interact (e.g., oldest child tends to be more authoratative).
- Different siblings may receive the same treatment but interpret it differently ("Mom always loved you best"); must take into account not only what parents do but how it is perceived by different sibs.
+ Genetics determines environment: some genetically influenced traits will promote exposure to a particular environment
- More intelligent parents have more books in their homes, an environmental factor from the perspective of the children in that home.
- More intelligent children elicit more intellectual stimulation from parents (an environmental factor).
- Musically talented people tend to be exposed to more music.
+ Additional reading:
- Buss (1999) Foundations of Evolutionary Psychology. Allyn & Bacon: Boston
- Plomin (1990) Nature and Nurture: An Introduction to Human Behavioral Genetics. Brooks/Cole: Pacific Grove CA.