Twin studies as evidence in the nature-nurture debate
A Report
Introduction: A general overview of the nature-nurture debate
The nature-nurture debate is concerned with the contributions of hereditary and
environmental factors to development (Bee & Boyd 2004, p.3). On the nature side of the
debate are the nativists, who see biological imperatives as the very core of human behaviour.
Hereditarians essentially argue from the position that genes are dominant over environmental
influences (Gross 2009, p.68). The origin of nativism is associated with the works of such idealist
and rationalist philosophers as Plato, Descartes and Leibniz, who defended some version of
innatism; the doctrine claiming that certain knowledge is a priori, with particular concepts
being present at birth (Garvey & Stangrom 2012). In psychology, this position was promoted for
instance by Hall, who contended that development was determined by genetically coded
physiological factors (Arnett 1999, p.317). He was greatly influenced by Lamarck’s evolutionary
theory, which suggested that beneficial, helpful traits acquired by an organism could be passed
down to the next generation in forms of acquired characteristics. This idea of ‘accumulated
experience’ led Hall to devise his psychological theory of recapitulation, which postulated that
the history of mankind overtime had become part of the human genetic code.
Empiricists, on the other hand, work from the assumption that experience is far more
important in shaping human behaviour than nature. Empiricism, which sees human behaviour
as learned as opposed to biologically pre-programmed, is mainly associated with the works of
such British philosophers as Locke and Hume (Delius, Gatzemeier, Sertcan & Wunscher 2013).
Locke (1960) argued that humans are born Tabula rasa with no in-built content. According to
him, all knowledge is a posteriori, acquired purely through experience. In psychology, this
position was advocated for example by the behaviourists; among them John B. Watson, who
launched a broad attack on naturalistic explanations of behaviour with his theory of classical
behaviourism (Davidson & Neale 1998, p.39).
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, Traynor and Singleton (2010, p.196) argued that the nature-nurture dualism is merely ‘’ a
false dichotomy’’. Psychologists and scientists today tend to take an inter-actionist, instead of
an extreme either/or approach, acknowledging the importance of both nature and nurture
(Bradshaw & Ellison 2009).
Twin studies cited in favour of genetic influence
To uncover the separate contributions of genetic and environmental factors, quantitative
genetics have employed family resemblance studies including twin studies. Although
fraternal/dizygotic twins only have a 50% genetic similarity, making them no more alike
genetically than any other pair of siblings, identical/monozygotic twins share 100% of their
genes as they develop from the same fertilized egg (Deary 2001, p.69). Twin studies, especially
those of monozygotic (MZ) twins reared apart, are often cited to provide evidence for the
contribution of genetics to various psychological traits such as intelligence as well as
psychopathological conditions such as schizophrenia, bipolar disorder and autism spectrum
disorder.
Bouchard and McQue (1981) conducted a meta-analysis of 111 familial studies of
intelligence and concluded that the closer the genetic link, the stronger the correlation
between IQ scores. They found that the weighted average correlation for monozygotic twins
brought up together was + 0.86., MZ twins reared apart had a correlation of + 0.72, which was
still higher than that for dizygotic twins reared together. This finding seems to support the
notion that this trait is substantially influenced by genetics. The Minnesota Study of Twins
Reared Apart (1990, as cited in Kamin & Goldberger 2002) yielded similar results, having
reported that about 70% of the differences between IQ scores is attributable to heredity.
Furthermore, Pederson, Plomin, McClearn and Friberg (1988) conducted a twin study to assess
and compare various personality traits in both identical and fraternal twin pairs reared together
and apart. Based on their findings, the researchers concluded that about 30% of individual
differences in neuroticism, and 41% in extraversion are due to heredity. Hereditary
predisposition, also known as diathesis, for schizophrenia has also been documented with a
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