Methodology Flashcards
What can we conclude about genetics from animal studies? (a priori -> eniviron)
- Behavioural traits inherited, a priori, indicating inheritance. A priori since we need to account for the environment.
Name advantages of animal studies.
- high control, thus can dissociate genetic effects from confounding environment effects (e.g. cross fostering to prevent maternal effects)
- Mice are a cost-effective and efficient research tool. They are small, they reproduce quickly, and they are relatively easy to handle and transport.
Animal study caveats.Genome differences (e.g. X-inactivation). Inbred (reasons + limitations). Complex psychiatric disorders.
- While structurally similar, the rat and human genomes differ significantly in their regulation, expression, and post-transcriptional modification, for example, species-specific differences in X-inactivation.
- To ensure physiological homogeneity and homozygosity, experimental rodents tend to be of inbred backgrounds, while humans are generally outbred and thus show large allelic variability, having differential modifying effects on the gene being studied.
- Useful for basic mammalian brain development/function studies, but cannot fully explain pathogenesis of complex psychiatric disorders which often exhibits symptoms unique to humans.
How do DZ vs. MZ twin studies claim to be advantageous over relatedness studies? What is crucial in twin studies?
Ostensible claim to control for environment that profoundly confound family studies. Crucial is comparison between DZ and MZ, without one or the other, data is meaningless.
Indirect evidence of genes and environment role, Thompson et al. (2001; MRI), Blockland et al. (2011; n-back)
- MRI structural analysis of grey matter. No similarities in unrelated. DZ = +similarity, MZ twins = greatest similarity, in some regions to impressive degree. But not completely the same.
- Large no. of young, adult MZ and DZ twins, n-back working memory task. BOLD functional activation sig. greater correlation in MZ than in DZ, but again nowhere near same patterns of performance.
Method issues: 1/85. Pre-natal.
- 1/5 conceptions = twins. 1/85 live births = twins. Twins tend to be lighter @ birth, increased risk for perinatal complications/death. More likely to result from ART-> epigenetics, aberrant DNA methylation.
Method: EEA assumption, implications. MZ more or less similar? Chorionicity, exsanguination.
- EEA: Rather than be treated more similarly, actually tend to differ more in certain aspects. Tend to differ more in birth weight due to monochorionicity, the net effect of which is that one twin receives a lot of blood relatively speaking, while the other remains exsanguinated. However, whether twins are monochorionic or dichorionic isn’t normally a factor that is known or considered in twin studies, despite potential impact of anoxia from this transfusion syndrome, e.g.in IQ.
Method: DZ more similar than other 1st degrees.
- DZ twins should be exactly as similar as non-twins, but born at the same time, more likely to share environment.
Method: Attrition and non-participation rates, implications: representative, pattern distortion (Moffitt & Caspi, E-Risk Taylor, 2004).
- In longitudinal epidemiological studies using birth registers, usually to have non-participation rates of 30% or so. Attrition over course of study may further increase loss.
- Those left may not be representative. Those initially missing disproportionately likely to have psychopathology in offspring or parents.
- Attrition biases distort patterns of assoc which can mislead inferences of nature-nurture interplay.
- Illustrated in general study of language, twins born to young mothers who were socially disadvantaged were substantially underrepresented (latter correlated with certain phenotypes, esp. certain psychiatric disorders such as SCZ).
- Quantitavely analysed by E-Risk studies, finding a sig bias to findings that were directly a result of sample attrition.
- Taylor (2004) found that the effect of this was a failure to detect shared environmental effected, an inflation of additive genetic effects and nonshared environmental effects, and sometimes the spurious identification of rating contrast effects or non-additive/synergistic genetic effects.
Reared-apart MZs. V rare.
- Constructive replication of findings from adoption and reared-together studies. If there is sig. psychological similarity 2 members of MZ twin pairs even when reared in separate homes from infancy, must be genetic?
Findings of reared-apart MZs.
- Similarity ranges from moderate (for social attitudes) to strong (for IQ), implicates genetic influences of varying magnitude across a range of psychological outcomes. -
- Twin similarity is never perfect, implicating environmental influence, which for social attitudes and interests account for majority of the phenotypic variances.
- Except for IQ, reared-together MZ-twins not much more similar than reared-apart MZ twins, i.e. common rearing may not have substantial effect on individual differences in the domains represented.
Reared-apart MZs cons.
- Since so rare, question is what’s special about circumstances leading to separation? May not necessarily happen at birth? Why would they volunteer for research together? Rutter (2007) argues that while some may be brought up in different circumstances, most would be in similar environments.
- Twins share common prenatal environment, different to DZ which can have profound effects as explained before.
Function and methodology of adoption studies.
Give rise to separable genetic and environmental relatives, assumption is that resemblance between genetic relatives due to genes, resemblance between environmental relatives due to environment.
- Either parent-offspring or sibling comparisons.
Adoption study findings and conclusions:
Despite positive findings for diverse traits (IQ, criminality, alcoholism, SCZ) many researchers also failed to find sig. resemblance among relatives. E.g., being reared by SCZ parent, criminal, or an affective disordered parent was not assoc. w/ increased offspring risk as long as they were not genetically related to rearing parents.
- Conclusion: while environmental effects on human behaviour are pervasive and strong, they appear primarily to contribute to differences rather than similarities among reared-together relatives (Plomin & Daniels, 1987).
Methodological issues: representative, selectivity, prenatal effects, lack of adoptions.
- adoptees and adopters are broadly representative.
- confounds of selective placement, e.g. high IQ child being adopted by high IQ parents usually not the case.
- prenatal effects may be more commonlace, but with subtler effects. The resemblance between adopted siblings, esp. in twins may reflect common prenatal influences.
- Lack of adoptions: difficult due to declining number due to contraception, education, abortion, changing social attitudes to single mums etc. Maybe adoption cases atm are more severe?
