PSYC2002 Study Guide - Final Guide: Quantitative Trait Locus, Twin Study, Natural Experiment
Biological Foundations & Motor Development
Genetics and Developmental Psychology
• There is natural variation in the human population: physical (eye colour), cognitive (IQ), and behavioural
(personality).
• Genetics argues for nature.
• Psyc interfaces with genetics: genotype + environment = phenotype
Genetics: Basics
• 46 chromosomes - 22 pairs of autosomes, 2 sex chromosomes
• On each chromosome there are set of genes, which code for organism functions either in isolation or
through interaction with other genes.
• We inherit traits from our familial group.
• Some instances of inheritance are simple, i.e. involving one gene.
• Others are more complex - polygenic inheritance.
• Behavioural genetics - aims to determine the contributions of nature and nurture to behavioural
diversity.
• Heritability - the proportion of a given behaviour that is due to genetics rather than environmental
factors.
• Two types of natural experiment allow us to tease apart genetics from environment: twin studies
1. Identical (monozygotic) twins - splitting of a single fertilised egg. Same environment/same
DNA. Shares 100% of genetic code.
2. Fraternal (dizygotic) twins - separate fertilisation of two eggs. Same environment/different
DNA. Share 50% of genes on average.
• Logic: Identical twins - Fraternal twins = (Genetic + Environment) - Environment.
• If identical twins are more similar on a given behaviour than fraternal twins, then the behaviour has a
heritable component.
• Heritability coefficient: H = 2[r(Identical) - r(Fraternal)]
• Problem with twin studies - can we assume that each type of twin shares the same environment?
• There are reasons to believe that their environment is altered by their identity:
o People treat identical twins and expect them to behave more similarly than fraternal twins.
o The manner in which each type of twin is socialised is likely to lead to similarities in behavioural
outcomes.
• Most stringent test: identical twins separated from birth.
Strengths and limitations of twin studies
Strengths
Limitations
• Proportion of shared
genes is known (on
average).
• Natural experiment =
ethical way to
investigate
heritability.
• Over-assume environmental factors are the same for each member of a twin pair.
• Assume environmental factors are just as similar for fraternal twins as for identical
twins; this is unlikely.
• Ignores gene-environment interactions, eg: genes drive physical attributes which
can influence the environment a person experiences.
• Only apply to a given sample; proportion of variation that can be attributed to
genetics is influenced by variation in environment.
Heritability in Development
• 60% of variation in IQ in a given population is due to differences in their genes.
• Therefore, heritability estimates say nothing about the contribution of genetics and environment to a
trait in the individual.
• Heritability interacts with the environment.
• Haworth et al. (2009): heritability of IQ increases across development:
o In young children (2-4yr): E > G
o Adolescents: G > E
• Why? We actively construct our environment - the phenotype-environment correlation.
o We have increasing control over our experiences as get older.
o Sharing similar genetics may lead children to seek out similar environments (niche-picking).
• Environment constrains heritability.
• Turkeimer et al. (2003): heritability of IQ varies with SES:
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Document Summary
There is natural variation in the human population: physical (eye colour), cognitive (iq), and behavioural (personality): genetics argues for nature. Psyc interfaces with genetics: genotype + environment = phenotype. 46 chromosomes - 22 pairs of autosomes, 2 sex chromosomes: on each chromosome there are set of genes, which code for organism functions either in isolation or through interaction with other genes. Two types of natural experiment allow us to tease apart genetics from environment: twin studies. Identical (monozygotic) twins - splitting of a single fertilised egg. Shares 100% of genetic code: fraternal (dizygotic) twins - separate fertilisation of two eggs. Logic: identical twins - fraternal twins = (genetic + environment) - environment. If identical twins are more similar on a given behaviour than fraternal twins, then the behaviour has a heritable component: heritability coefficient: h = 2[r(identical) - r(fraternal)] Proportion of shared genes is known (on average): natural experiment = ethical way to investigate heritability.
