Everything is at least a little bit heritable

• Everything is at least a little bit heritable (that is, more closely genetically related individuals are more similar phenotypically). • Nothing is 100% heritable (even identical twins are a little different from one another on anything we can measure). • Heritability is caused by many genes of small effect (there is no “gene for” in psychology, human behavior, or indeed ANY trait that commonly varies between people). • Correlations between phenotypes are partly due to correlated genetic/heritable influences. • Heritability increases through development (at least through middle adulthood). • Trait stability or consistency across time within an individual is due to stable genetic influences. • Even things we think of as “the environment” show non-zero heritability (example: divorce). • Correlations between phenotypes and environments are partly due to correlated genetic/heritable influences. • Most environments are not shared between people, even siblings or twins raised together. • Abnormal is normal (influences on extremes that we label as “disorders” versus normal-range traits are not different in kind, just amount). • Interpretation of results means understanding the strengths and weaknesses of choices made in the design of a study (for example: how to measure a phenotype, how participants were selected or excluded). • Phenotypes (means, variances, and presentations) change over development – we cannot use the same measures or interpretations without knowing the developmental context.
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