Twins and Adoption their Effects on Criminal Behavior

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Cesare Lombroso, an Italian doctor of the 1870's, determined that some prisoners were genetically different to the general population - their biology made them aggressive and/or born to be criminals (Hollin, 1992; Goldsmith, Israel and Daly, 2003: 65). It wasn't until some years later that twin, then adoption, studies lent some credence to Lombroso's claims. However, not all twin and adoption studies support Lombroso's claims and there have been many detractors of twin and adoption studies themselves and of their results. So, what are twin and adoption studies and what can or can't they tell us about criminal behaviour?

A Twin Study looks at the similarity and difference between identical/single egg (MZ) and non-identical/two egg (GZ) twins. As MZ twins share all their genetic information, everything that is genetically encoded should be the same in each twin regardless of whether the twins share the same environment. In DZ twins there should only be similarities that would occur with any siblings (Hollin, 1992). Twins who have not shared the same environment offer researchers a valuable opportunity to contrast genetic variation and environmental differences.

Adoption studies generally focus on siblings/twins who were reared apart, contrasting their differences and similarities with siblings who are raised together (Plomin, 1989). If the child's behaviour and criminal propensity is most like that of their birth parents, then there is a strong case for heredity. Environment would be shown as the major influence if the child is more like their adopted parents (Hollin, 1992). Both twin and adoption studies are used to describe concordance (alikeness) and variance, of heritability.

There is much argument about what genes actually do determine. If criminal behaviour, as all behaviour, is determined by the brain and the brain's development is determined by genes, then all behaviour must be genetically encoded (BMA, 1995; Lawson and Heaton, 1999). Gene based biological theories (people are born criminals) became unpopular in the last century but have reappeared with advances in brain imaging - though now seen as neurological studies (AIC, 2002; Goldsmith et al, 2003: 68).

Without resorting to medical imaging, twin and adoption studies have also resurfaced in the past two decades and many researchers appear to have a lot to say about genetic influences on criminal behaviour and lifestyle choice. There also appears to be a lot of research into the style of parenting, the type of environment and the way they meld with genetics to determine criminality, amongst other expressions of self. Some researchers appear to specialise in certain areas of twin/adoption research whilst others concentrate on environmental influences. According to Plomin (1989), it is difficult but necessary to separate the influences of genes and the influences of environment.

The interest in the link between genes and intelligence moved toward an increased interest in determining the link between genes and criminal behaviour, with publication of a book by Wilson & Herrnstein in 1985, claiming that biology affects propensity for criminal behaviours (Plomin, 1989). However, it appears that not all researchers agree.

According to Gau, Silberg, Erickson and Hewitt, 1992; Rutter and Renshaw, 1991; Simonoff, 1992, evidence exists to show there is some higher incidence of conduct disorder in twins, when compared to singletons (Slutske, Heath, Dinwiddie, Madden, Bucholz, Dunne, Statham, & Martin,1997). Also, Rutter and Redshaw, 1991 and Simonoff, 1992, found that twins, on average, have slightly lower intelligence and more difficulties with language than a single-born. If lower intelligence (Lawson & Heaton 1998), difficulties with language, conduct disorder and other antisocial behaviour in the young, are precursors to criminal behaviour, then twins appear more likely to become criminals (Vold, Bernard and Snipes, 2002).

When Wilson & Herrnstein (1985) suggested that young delinquents who become criminals as adults might have had a genetically determined tendency, their findings were supported by research from Rowe and Osgood (1984) who found that 60% of the influence on delinquent behaviour came from genetic factors, approximately 20% of the influence came from the peer group chosen and the other 20% of the influence was based on non-shared environmental sources (Hollin, 1992). In Plomin 1989, several twin studies of adult criminal tendencies showed MZ and DZ twin concordances of 69% and 33%, respectively. Mednick, Gabrielli, & Hutchings (1984), found that adoption studies have shown some consistency with the hypothesis that genes influence adult criminality, although this evidence is not as striking as in the twin studies (Plomin, 1989). Gottesman, Carey, and Hanson (1983) noted that six twin studies of juvenile delinquency showed an 87% concordance for MZ twins and 72% concordance for DZ twins. This apparently indicates a slight genetic influence but substantial environmental influences on behaviour (Guze, Earls and Barrett, 1983).

Farrington, Barnes, & Lambert, 1996; Farrington, Jolliffe, Loeber, Stouthamer-Loeber, & Kalb, 2001; Rowe & Farrington, 1997 all confirmed that there is a familial concentration of crime within the general population. In general, fewer than 10% of the families in any community account for more than 50% of that community's criminal offences.

Back in 1930, Lange examined 30 male prisoners and found that of those prisoners who were MZ twins, 77% of them had a criminal for a brother. In the case of the DZ twins, only 12% of the brothers had a criminal record. Lange thus asserted that there was a genetic influence in play (Burke, 2001). Then in 1974, Crowe used an adoption study to look at the rate of criminal behaviour in young adoptees whose birth mother was a criminal. Crowe found that almost 50% of the adoptees, whose mother had a criminal record, had a record of criminal behaviour themselves by age 18. In the control group, only 5% of adoptees had criminal records by age 18, if their birth mother was not a criminal (DiLalla, 1991; Hollin 1992).

In 1997, Hutchings and Mednick studied male adoptees and discovered that 85.7% of males with a criminal or minor offences record, had a birth father with a criminal record. They also noted that young male adoptees without a criminal record, had a criminal father 31.1% of the time (DiLalla, 1991;Burke, 2001).

If twins are brought up together, do they have the same friends? Are their associations the reasons that they exhibit delinquent behaviour that could lead to crime? In their 1983 study, Scarr and McCartney were able to confirm that twins often chose the same friends and that this was genetically influenced (Slutske et al, 1997). Rowe and Osgood supported Scarr and McCartney's findings in part, when they noted that not only was there a link between peer group choice and antisocial behaviour but that choice of friends/playmates was genetically linked (Slutske et al, 1997). As twins appear to choose the same friends, do they also choose the same behaviour? According to Rowe (1985) and Carey (1992) twins do influence each other's behaviour and not only do they imitate their twin's behaviour but they also engage in delinquent behaviour as a pair. If a twin study does not account for twin imitation and shared behaviour, then the figures extrapolated from the research might be biased and overestimated (Slutske et al, 1997).

The family concentration of antisocial behaviour could be explained by a genetic influence on antisocial behaviour, but it could just as easily be explained by non-genetic, social transmission of antisocial behaviour within families but the causes of such behaviour are not yet fully understood and twin studies that can't or don't delineate between genetic and environmental influences, are not helpful (Moffitt, 2005).

Gene and genetic transfer studies had become unpopular and often refuted but in the last 15 years or so, study into the chemistry of the brain has resurfaced. Researchers have again been trying to determine if genes or chemistry decides the propensity for an individual's aggressive and/or criminal behaviour. (AIC, 2002) Further twin studies might or might not have a place in future research. However, it does appear that there is a strong connection, though not always well-defined, between criminal parents and criminal behaviour in offspring. If paired with gene imprinting from blood samples (to determine true zygosity) and brain chemistry analysis, future twin and adoption studies could continue to make a valuable contribution to studies of criminal behaviour (DiLalla 1991).


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