The study of criminal behaviour relies on the general notion that, there are certain acts consistent across the majority of societies that can be classed as criminal. Furthermore, these actions must be closely related to delinquent behaviour and even to the extent of being immoral. To identify characteristics of those who exhibit criminal behaviour we need to use conventional methodology consistently used in personality research. This involves finding individual differences in trait and response. In this case these factors need be identified between criminals and non-criminals and this has been a common approach from all different perspectives of psychology regarding criminal behaviour.
Furthermore, to argue this statement that criminal behaviour is largely determined by genetics, the strengths and empirical evidence supporting genetic-based theories will be assessed. For comparison, the proven effectiveness of genetic-based theories in determining a criminal personality will be assessed along with other potential causes of criminal behaviour.
One of the more popular genetic-based theories for criminality comes from Sheldon who proposed the idea that there are three variations of body type (somatotypes). Furthermore, these three body types are argued to have distinct traits which define their personality. The Mesomorphic' body type is stated as being broad /muscular with a competitive, adventurous and more interestingly; aggressive personality. The Ectomorphic' body type in contrast is fragile/thin with a restrained and introverted personality. Finally, the other body type is Endomorphic' which can be described as soft/round with a sociable and outgoing personality. All of these body type/personality descriptions were the product of his own research (Sheldon,1942).
The basis of this theory in regard to criminal personality makes the assumption that those with a mesomorphic body type are more prone to delinquency and criminal activity (aggressive personality). There is a wealth of empirical evidence to support this theory including a study by Sheldon himself who used a sample of over four-hundred males in criminal rehabilitation. From the results of this study he found an overwhelming majority of the sample taken were mesomorphs. In contrast, there was a distinct lack of ectomorphs who are portrayed as having a personality, almost directly opposite to that of individuals with a mesomorphic body type (Sheldon, 1949).
These finding were followed up by a later more developed study comparing personality types between five-hundred non-delinquents and five-hundred delinquent individuals. These were matched for age, intelligence, residence andethnic background. The findings supported the above mentioned study with over 60% of delinquents being identified as being mesomorphic as opposed to only 30% of non-delinquents (Sheldon & Glueck, 1956).
Although there are many studies which support Sheldon's constitutional theory there have been replications which have proven somewhat inconclusive. There are arguments over the credibility of Sheldon's studies; most notably, the delinquent groups were subjectively selected without the basis of legal definitions. Furthermore, a study aimed to replicate those
previous, but reclassifying delinquents using legal criteria. This study found a difference far less significant between different body types. However, delinquent youths still showed higher ratings towards a mesomorphic body type. (Cortes & Gatti, 1972).
Considering all evidence, it is clear there is a strong link between the mesomorphic body-type and the development of a criminal personality. However, the credibility of Sheldon's studies can be questioned regarding the defining of body-types. Sheldon himself admits there are hybrids between body-types and so the definition of the mesomorph may have been
manipulated to fit into theory. This idea is supported by findings in other studies which have shown less significance towards body-type and delinquency (Cortes & Gatti, 1972).
Ultimately, it's difficult to argue this theory is solely genetic-based as body-type may have an environmental influence. For example, the way these people are perceived by the police and criminal justice agents may make them vulnerable to the justice system. Furthermore, mesomorphs from an early age may find using aggression is an easy way to get what they
want using their muscular build to their advantage; thus being conditioned into delinquency in later life. Ultimately, the mesomorphic body type may have more success in crime and individuals with this body may be likely included in gangs delving in criminal activity.
There are numerous theories which claim the criminal personality has a hereditary or genetic component. Lombroso, a criminal anthropologist was one of the earliest theorists who believed criminal behaviour had a genetic component unlike that of their non-criminal counterparts. Furthermore, there was a genetic element formulating the idea that a criminal is
born, not made. In addition, Lombroso argued that indirect heredity' influences can create a criminal personality by means of interacting with other degenerates such as the insane. To gather research to support the genetic influence in the creation of a criminal we need to rely on twin and adoption studies. Firstly, twin studies, particularly monozygotic twins (Identical) allow us to assume concordance rates between them, go some way to supporting genetic theorists about criminals.
Twin studies aim to compare the difference in concordant rates between monozygotic twins (MZ- indentical) and dizygotic twins (DZ- genetics comparable to normal sibling). The assumption behind this comparison is the idea that both sets of twins share a similar environment and so any discernable difference in concordance of delinquency can be credited to genetics.
Early studies in the area found extremely significant concordance rates for MZ twins in contrast to DZ twins. For example (Lange, 1929) found 77% concordance rate for criminal behaviour between MZ twins compared to a 12% rate in DZ twins. Similarly, (Rosenoff, 1932) found a concordance rate of 68% between MZ twins as opposed to a concordance rate
of 10% in DZ twins. These findings put great emphasis on the importance of genetic influences on criminal behaviour. However, the credibility of these early studies was heavily criticised for small sample sizes with the above studies having a sample size of only fifty MZ twins and only seventy-seven DZ twins collectively. Additionally, the earlier studies did not
have the advantages of DNA to identify the ziygocity of twins and relied on face recognition and other unreliable methods. Ultimately, advances in technology and understanding on DNA were capitalised in later studies to improve credibility.
Moreover, a good example of a later study examined influences on delinquency with the focus on three factors and not solely on genetic concordance. These factors were twin type (MZ or DZ), environmental influences (i.e. poor background) and environmental influences for the individual twin (i.e. peer group). The sophisticated design matched sample groups of
MZ and DZ twins to be similar. The goal of this method was to reach a more concrete conclusion on the dominance of genetic factors over environmental. The study concluded that genetic factors had a 60% influence on criminal behaviour. Furthermore, the shared twin environment and the individual twin environment each only accounted for a 20% influence
(Rowe & Osgood, 1984). These statistics emphasise the role genetics play in the criminal personality, this concept is widely supported by many reviews of studies in the area of criminal psychology. However, there are limitations to twin studies such as a selection bias towards MZ twins who are more similar in terms of personality and the notion that MZ twins may share a more similar environment than DZ twins (i.e. parents and peers may treat MZ twins differently).
A possible solution for this problem comes in the form of adoption studies using a sample of children who have a biological parent, or biological parents with a criminal record. In one particular study, a sample of adopted children whose biological mothers had a criminal record were selected and matched against adoptees whose biological mothers did not have a criminal record. The findings of this study concluded that 50% of adoptees whose biological mother had a criminal record, had one themselves by the age of eighteen compared to only 5% in the other group. With the absence of family influence (environmental) the findings give strong evidence in support of genetic influence (Crowe, 1974). However, likewise with twin studies, adoption studies are still not a conclusive representation of genetics and criminal behaviour.Moreover, it is impossible to rule out the role of environmental factors in the causation of criminal behaviour. This is evident in another study which took criminal records of both biological and adoptive parents into consideration.
The results confirmed that as well as a genetic influence there is also a significant environmental influence. Firstly, the adoptee with non-criminal biological parents and criminal adoptive parents were over 10% more likely to receive a criminal record than those whose biological and adoptive parents who had no criminal history. In addition to this, adoptees that had both a criminal biological and adoptive parents showed the highest rates of receiving a criminal record in comparison to all other combinations in this study (Mednick et al., 1983). Ultimately, environmental factors cannot be ignored in this particular topic.
To fully explain the influences of criminal behaviour it is evident from above assessment of biological explanations that the role of genetics cannot be fully accountable. Furthermore, although biological theories and research have strong support for genetic influences on criminal behaviour, the findings have also suggested that social factors are likewise significant. As stated previously, although studies conducted for biological theories provide evidence for a genetic component of criminal behaviour they simply can't explain it solely with these explanations. Moreover, arguments can be made regarding constitutional theory as the findings could be a result of social interaction concerning their body rather than a specific genetic code. Furthermore adoption studies using a sample of biological and adoptive parents with or without criminal records proved both genetic and social influences.
In regards to the original statement it is impossible to explain criminal behaviour simply through genetics and so the role of social influences needs to be addressed. Firstly, the role of parenting is seen as an important factor. The findings of a collection of case studies investigating parenting in delinquent' found that their methods were consistently lax, harsh and overly punitive (no control) (Patterson, 1988). In other research there is evidence to suggest that intelligence may be an important factor in the causation of delinquency.