Heritability (h²)
How much of the differences we see in a trait come from genetic differences
Shared environment (C)
Influences siblings share (home, parenting style, neighborhood) that make them more alike.
Nonshared environment (E)
Experiences siblings don’t share (different friends/teachers, random events); also includes some measurement error.
Variance
How much people differ from each other on a trait.
Causation
A factor directly produces a change in an outcome.
A/C/E model
A = genes, C = shared environment, E = nonshared environment; together explain variance in a trait.
MZ vs DZ twins
MZ share ~100% of genes; DZ share ~50% on average.
Why is “correlation ≠ causation”?
Two things moving together doesn’t prove one makes the other happen.
Naturalistic approach
Describe the world as it is, not as we wish it to be.
Behaviorism
A view that psychology should only study observable behavior, not unobservable stuff.
Naturalism vs behaviorism — key difference?
Naturalism allows studying unobservables if measured well; behaviorism avoids them.
Naturalistic fallacy
Saying “it’s natural, so it’s right.” (Is → ought mistake.)
Moralistic fallacy
Letting what you want to be true decide what you claim is true. (Ought → is mistake.)
Confirmation bias
Favoring evidence that supports your preferred view and dismissing the rest.
GWAS
Genome-Wide Association Studies: test whether many small DNA differences together predict an outcome.
Polygenic
Many genes, each with tiny effects, add up to influence a trait.
What is a trade-off?
Choosing one benefit usually costs you something else (time, energy, risk).
Future discounting
Valuing rewards now more than the same reward later.
Turkheimer’s 1st Law
All human behavioral traits show some genetic influence.
Turkheimer’s 2nd Law
Shared family environment usually matters less than genes.
Turkheimer’s 3rd Law
A large chunk of differences isn’t explained by genes or shared family; nonshared environment is big.
What does “heritability > 0” actually mean?
Genes explain some of the between-person differences; not that genes fully control a trait.
Why doesn’t “family resemblance” prove environment?
Families share both genes and homes; you need designs that separate them.
Gloomy prospect
Many outcomes are shaped by random, one-off events we can’t measure well, so prediction is limited.