Title:
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The classical twin study in human behaviour genetics
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The aim of this thesis is to show the variety of information
which can be obtained from sensitive analysis of data on MZ and DZ
twins reared together, the most popular design in human behaviour
genetics.
Initial theoretical considerations suggest the range of
models it is possible to fit to the data. We introduce a method of
calculating power based on the non-central chisquare distribution. This
allows calculation of sample sizes required to reject false hypotheses
with given probability and it is shown that even in some simple
situations, very large sample sizes are required for reliable rejection.
Our calculations suggest that the power of the three twin studies
analysed to discriminate between simple alternative models, is rather
low.
A twin study of examination performance is able to reject
environmental models for variation in most variables and detects large
heritabilities for both scholastic achievements and IQ. Significant
heterogeneity of gene action is found for English and Mathematics
suggesting specific genetical variation for those subjects.
A larger study of sexual and social attitudes and
personality finds several differences in patterns of variation between
the sexes. Many genotype-environment interactions are demonstrated for
certain scales of measurement but rescaling the data to remove them
makes little difference to the results of genetical analysis in
most cases. Evidence for the genetical effects of competition
between twin sisters is found for the trait of Sexual Satisfaction and
differences between males and females in the relative importance of
cultural and genetic variation are found in Libido, Masculinity
and Toughmindedness. There is directional, but not significant
evidence of higher heritabilities for personality traits in men.
Because of variation in the age at which different genotypes begin
sexual activity the heritability of age of first sexual intercourse
increases with age itself. Surprisingly, there is no evidence of
cultural effects influencing this trait.
The behaviour of twin boys interacting with their parents
was recorded by ethological observation in both twins and singletons.
Most of the measurements were too unreliable to hope to detect genetic
variation with such small samples, the only exceptions being child
speech and a measure of rudimentary conscience. However, some new
evidence against arbitrary parent-initiated treatment inequalities
for MZ and DZ twins provides further support for the validity of the
twin method.
A method for the genetical analysis of covariance structures
is developed which allows simultaneous maximum likelihood estimates of
general and specific variation from the various genetical and
environmental sources. The method is illustrated on some twin
data for cognitive abilities.
Possibilities for future research arising from the results
of each of these twin studies are discussed.
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