“Shared versus Non-shared Environment Distinction…The Moderating Effect of Development… Gene-Environment Correlation… Family Socialization: A Behavioral Genetic Perspective… Gene-Environment Interaction II – UPDATED… Genetics and Aging – UPDATED… Supplemental – Epigenetics and Twins…”
Module A: Shared versus Non-shared Environment Distinction
Module B: The Moderating Effect of Development
Module C: Gene-Environment Correlation
Module D: Family Socialization: A Behavioral Genetic Perspective
Module E: Gene-Environment Interaction II - UPDATED
Module F: Genetics and Aging - UPDATED
Module G: Supplemental - Epigenetics and Twins
Module A: Shared versus Non-shared Environment Distinction
Some of the most intriguing research in behavioral genetics is not about the genetics about about environmental influence.
Environmental influence can be:
Shared environmental influence (C) – factors shared by siblings/twins raised together e.g. socioeconomic status, parents’ approaches, neighbourhood and school districts etc;
Non-shared environmental influence (E) – factors not shared by siblings/twins raised together e.g. peer group, accidents, differing treatment by parents etc.
Note: it is about the effects, not the factor per se i.e. there can be a nuance – poverty might be a shared environmental effect potentially, but if different twins/siblings respond differently, it would be non-shared.
General Cognitive Ability – environmental influence largely shared;
In general, most psychological and psychiatric traits seem to be non-shared environmental influence.
Detailed explanations of two studies: that the non-shared environment seems to be the predominant environmental influence.
Exceptions (i.e. show significant shared environmental influence) are general cognitive ability, anti-social/rule-breaking behavior, and social attitudes such as religiousness.
This might in fact reflect where parents in general place their emphasis in raising children.
Recap: most studies do not support strong shared environmental effects on behaviour except in General Cognitive ability, rule breaking behavior, and social attitudes and religiousness.
A lot of behavioral genetic research on General Cognitive Ability is done with children and adolescents, and there is less data on adults; this matters because genetic and shared environment contributions might change with age.
In fact the former increases while the latter decreases; supporting evidence for this comes from
Cross-sectional studies of reared-together twins;
Longitudinal twin studies;
Cross-sectional and longitudinal studies of adopted siblings;
Longitudinal studies of adopted parent-offspring resemblance.
Detailed explanation of related studies of the above.
Pattern holds for social and political attitudes, and rule breaking behavior.
Hence these shared environmental influences are exceptions but they also seem developmentally limited i.e. to the time the siblings etc are staying together.
In summary: behavior is not determined by genetics only; environmental influences exist, and it is usually the non-shared environment (and for traits with shared environment influence, it is limited to a period of living together).
Gene-Environment (G-E) correlation is a statistical correlation the magnitude of genetic effect and environmental effect.
Three types of G-E correlation.
Passive G-E correlation: parents who transmit genes that might promote the development of a particular trait are likely to also create a rearing environment that promotes the development of that same trait; it is “passive” because the child plays no role in this.
Reactive G-E correlation: the way we behave elicits certain reactions from others (i.e. our environment/experiences), and if that behavior is genetically determined, then the behavior and environment are correlated.
Active G-E correlation: because of our abilities and interests, we make choices that in turn shape the environments we experience, and if those abilities and interests are genetically influenced, then a G-E correlation is induced.
G-E pervasive: monozygotic twins experience more similar environments than dizygotic twins.
G-E correlation is important to psychology for three reasons:
Understanding parent-offspring correlations.
Behavioral traits are quite different from physical traits because of G-E correlation.
Understanding developmental changes in heritability (e.g. shared environment declines with age and heritability increases with age reflects passive and active G-E respectively, while reactive G-E is fairly constant through life).
Next module: family socialisation processes – findings on the small influence of shared environmental influence seems to contradict the idea we are socialised by the families who raised us.
Module D: Family Socialization: A Behavioral Genetic Perspective
Parental behavior and offspring function:
Parents who did well in school tend to have children who did well too;
Children of parents who smoke have higher risk of smoking;
Overweight parents tend to have children who are overweight too;
Children of parents who are professionals tend to have larger vocabularies;
Children who are anxious tend to have parents who are overprotective.
But are these correlation or causation?
Detailed example of association of smoking in mothers and ADHD in the child – to show we need more evidence such as sibship studies before we can begin to know if an association is causal or due to other factors.
Module E: Gene-Environment Interaction II – UPDATED
There has been a surge in Gene-Environment interaction (GxE) research.
Two differing perspectives on GxE: interactionism and statistical.
Interactionism (Developmental gene by environment interaction): gene and environment are so inextricably linked that it is meaningless to talk about their separate influence i.e. nature is inseparable from nurture and should be understood together.
e.g. blue eyes – meaningless to ask how much of the blue pigment came from the genes and how much came from the environment.
Statistical (Biometrical): genetic and environmental effects combine in a non-additive way to account for the variance/individual differences in a phenotype/trait
e.g. blue eyes vs brown eyes – the difference might be due to genes, to environment, or an interaction of both.
the environmental influence depends on the genotype or equivalently the genetic influence depends on the environment.
Diathesis-stress is an example of statistical gene-environment interaction e.g. PKU (phenylketonuria) – see first week’s modules – where the genetic effect depends on the environment, and the environment effect depends on the genotype.
This is important because clinicians can adapt environments of those who have risk genotypes, and is also the basis for personalised/individualized genomic precision medicine.
Landmark study of GxE involving serotonin transporter system and depression (introduced in Week 2) – discussion on:
the study;
how well the study has help up;
how it shows there might be uncertainty at the frontiers of science.
Genetics and aging is an emerging field of inquiry.
As one grows up, genetic factors seem to matter more from the time one is being raised to when one sets out on his or her own. What happens after that i.e. in adulthood?
Answered by using three questions:
Do twins become more or less similar with age?
How important are genetic factors for late onset disorders?
To what degree do genetic factos contribute to aging?
For the first question, it seems the genetic influences remain fairly stable.
For the the second question, it depends on the disorder (Alzheimer’s is highly heritable, Parkinson’s is weakly, and macular degenerates in in between).
Another way of defining aging is how long you. For this, no twin similarity in general for how long one lives, but if you are to live very long (80s-100s), genetics is probably very important.
Tremendous amount of research on twins and epigenetics.
Two main areas:
Estimating heritability of epigenetic markings (they are phenotypes);
Understanding why MZ twins (same genome) might be discordant (e.g. for different diseases).
On the first area, discussion on several studies using DNA methylation (first mentioned in Week 4) – note that there are not a lot of large scale studies currently.
Current research suggests epigenetic phenomena are probably partly heritable.
On the second area, discussion using different studies:
Beckwith-Wiedemann Syndrome: Epigentic phenomenon and methylation on gene on chromosome 11 for one twin but not the other, so one had the syndrome and the other did not.
Epigenetic differences over a lifetime: comparing similarities and differences in the chromosomes of twins at different ages – epigenetic markings can change over a life span (epigenetic drift).
Research in this field is still in the early stages and samples might be too small too, as well as challenges with tissue specificity, cause vs effect, and chronicity.
