Genetic Explanations for Aggression: Studies

Heritability of aggression

Sex Chromosome Abnormalities

Y-Chromosome Hypothesis

X-Chromosome Hypothesis

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Genetic Explanations of Aggression: Studies
Canter (1973) - Found a correlation of 0.14 for monozygotic twins (MZs)
reared together.
O'Connor (1980) ­ Found a correlation of 0.72 for the same kind of
population.
Miles and Carey (1997) ­ Suggest a heritability measure of aggression
of 50%.
Plomin et al (1990) ­ Found a small heritability for aggression. Found
average concordance rates for MZ twins was 0.32 and DZ twins
(non-identical) was 0.14.
Rhee and Waldman (2002) ­ Suggest the variability may be due (in
part) to the methods of assessing aggressive behaviour used in the
various studies.
o They did a meta-analysis of 51 twin and adoption studies.
o Found that there was a significant differences in the magnitude
of genetic and environmental effects according to how aggression
was measured.
o Also found that heritability varied according to the type of
assessment used: 39% for self-reported aggression and 53% when
it was reported by others.
Jacobs et al (1965) ­ found that the incidence of XYY syndrome among
inmates of institutions was approx. 3% whereas in the normal population
it is only around 0.1%.
o Further studies indicated that these men were taller, had higher
levels of testosterone and lower levels of intelligence than non
XYY people.
Witken et al (1976) ­ Conducted a study that looked at the importance
of the extra Y chromosome in determining aggressive behaviour. Their
sample included all males in the City of Copenhagan between 1944 and
1948.
They focused on the tallest 16% of this group. They then attempted to
visit all 4591 individuals to determine their chromosome type, they
managed to contact 91% of them.
12 were XYY
4111 were XY
16 were XXY

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Public records were also examined regarding height, IQ, socioeconomic status
and criminal activities.
o Their findings confirmed results reported previously, 9.3% of the
XY subjects had been convicted of one or more crimes, whereas
41.7% of XYY subjects fell into this category (5!).
o Found no significant differences between the (violent) offense
records of XY and XYY participants.
o Could not find a link between XYY syndrome and increased
aggressiveness. But they do support previous findings that XYYs
had lower levels of intelligence.…read more

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Maybe what is being inherited is not aggression itself, but maybe
an underlying characteristic (such as dominance/impulsivity)
that increases the probability of aggression.
o (Female convictions were too infrequent for the researchers to
make meaningful connections of parent and child convictions
from!)
Williams (1988) ­ States that a male's likelihood of criminal conviction is
related to the criminality of his biological parents. But, the contribution
of biology does not exclude the influence of the environment.…read more

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Theilgaard (1981) ­ Available evidence suggests no differences in
aggressive behaviour between XXY and XY males. XXYs seem to suffer
mental impairment which may account for the high rates of
institutionalization.
o They also reported the results of psychological tests and
interviews from the Witken et al (1976) study.…read more

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