For most of the twentieth century, criminology treated crime as a product of society: poverty, broken families, bad schools, and unequal opportunity. But not every expert agreed that social conditions told the whole story. In their influential book Crime and Human Behavior, James Q. Wilson and Richard Herrnstein argued that criminologists had focused so heavily on social explanations that they had ignored a large body of evidence pointing to biological factors – the traits we are born with or that develop very early in life – as real, independent contributors to criminal behavior.
This does not mean anyone believes people are simply “born criminal.” What the evidence actually supports is more modest and more interesting: some people inherit tendencies – in temperament, impulse control, and intelligence – that make them more vulnerable to becoming offenders when they are also exposed to difficult social conditions. Biology loads the gun; circumstances often pull the trigger.
What Twin Studies Show
One of the oldest tools for separating nature from nurture is the twin study. Identical (monozygotic) twins develop from a single fertilized egg and share virtually all of their genes. Fraternal (dizygotic) twins develop from two separate eggs and share, on average, only half their genes – no more genetically alike than ordinary siblings. If criminal behavior were purely a matter of upbringing, identical and fraternal twins raised in similar households should resemble each other equally in their likelihood of offending. If genetics plays a role, identical twins should resemble each other more.
More than a dozen studies, covering upwards of 1,500 twin pairs in the United States, Scandinavia, Japan, Germany, Britain, and elsewhere, have tested exactly this. Karl Christiansen’s landmark study of Danish twins born between 1881 and 1910 – tracked through police, court, and prison records – found that when one identical twin had a criminal record, the co-twin was considerably more likely to also have one, compared to fraternal twin pairs. Later research, including a large mail survey of Danish twins by the American criminologist David Rowe, confirmed the same pattern: identical twins were more alike not only in whether they offended, but in related traits such as attitudes toward risk-taking and rule-breaking, and in how much they were influenced by delinquent friends.
What Adoption Studies Show
Twin studies have a weakness: identical twins are usually raised in more similar environments than fraternal twins, which muddies the comparison. Adoption studies solve this differently, by looking at children separated from their biological parents at or shortly after birth and raised by unrelated adoptive families.
The best-known of these is the work of Sarnoff Mednick, who studied thousands of adopted boys born in Denmark between 1927 and 1947. Mednick compared the criminal records of each boy’s biological parents and adoptive parents against the boy’s own record as an adult. The results were striking: when a boy’s biological parent had a criminal record but the adoptive parent did not, the boy’s own risk of offending rose noticeably above average. When neither set of parents had a record, or when only the adoptive parent did, the risk barely moved. Writing in the journal Science in March 1985, Mednick concluded that the study “clearly demonstrates that some factor transmitted by convicted parents increases the likelihood that their offspring will be convicted of a criminal offense,” and that because this transmission runs through biological rather than adoptive parents, genetic factors must be involved.
It is worth being precise about what this does not show. No “crime gene” has ever been found, and researchers who conduct these studies are usually the first to say so. What seems to be inherited is not criminality itself but underlying traits – temperament, impulsivity, sensitivity to reward and punishment – that make certain environments far more likely to tip a person toward offending. Heredity and environment consistently turn out to matter together; neither one, on its own, is a reliable predictor.
The IQ Debate
A related and more controversial line of research concerns intelligence. A large study of army recruits tested with early intelligence exams found that those who went on to commit crimes scored, on average, lower than the general population, although the difference between offenders and non-offenders on these tests was modest, and the gap narrowed considerably in verbal versus performance measures.
Critics have long pushed back hard against reading too much into these numbers. Some scholars argue the tests themselves are culturally biased, favoring people from more educated, materially secure backgrounds – exactly the people least likely to end up with a criminal record for reasons that have nothing to do with intelligence. Others point out that low scores may reflect poverty and poor schooling rather than any innate deficit, and that poverty is already known to raise crime risk through entirely separate pathways. A widely cited review by the U.S. National Academy of Sciences concluded that whatever relationship exists between IQ and offending is not a simple, direct causal one, and cannot be separated from the social disadvantage that so often travels alongside it. Claims that have occasionally surfaced linking intelligence, race, and crime rates have no credible scientific support and are rejected by serious researchers in the field.
Temperament and Early Warning Signs
Personality traits associated with repeat offending are more consistent across the research than IQ. People who offend again and again tend to show a cluster of traits from an early age: impulsivity, a taste for risk and stimulation, unusual fearlessness, and difficulty tolerating frustration – often alongside genuine trouble expressing themselves in words, which can turn into aggression or rule-breaking as an alternative outlet.
One of the most cited studies here followed more than 500 children who had been referred, in the 1920s, to a child guidance clinic in the United States – the American sociologist Lee Robins tracked what became of them as adults. The children eventually diagnosed as displaying sociopathic traits shared an early warning pattern: truancy, theft, running away from home, lying, promiscuity, and a striking lack of guilt about any of it. Notably, these signs appeared well before age twelve, in most cases, and half of the children who would become the most seriously antisocial adults had shown large behavioral problems before they were even eight years old. A comparable study by John J. Conger and Wilber Miller, following a sample of delinquent boys from the same school system, found the same children had been flagged as behaviorally different as early as third grade – not yet delinquent, but already treating classmates and their rights with noticeably less regard, and showing little willingness to accept responsibility within a group.
Robins’ research also found these early problems were frequently accompanied by symptoms resembling what would now be called ADHD – difficulty sitting still, unusual and impulsive movements, and trouble regulating attention – sometimes visible years before anyone thought to look for them.
Risks Before and At Birth
Biological risk is not only about what is inherited; it also includes what happens in the womb and around birth. Premature birth and low birth weight, alcohol and drug exposure before birth, and inadequate prenatal care have all been linked to later behavioral problems. These factors are frequently tangled up with poverty – parents under financial stress are less able to access good prenatal care or afford treatment for substance use – which makes it hard to say how much of the effect is biological versus social. As one economist, Morgan Reynolds, put it in his book Crime by Choice: remarkably few serious discussions of crime give heredity and environment equal weight; the debate tends to swing dramatically toward one explanation or the other, when the honest reading of the evidence is that both play a part, and that a good deal of individual difference in vulnerability to crime is simply not attributable to either one alone.
Why This Research Remains Controversial
Biological explanations of crime carry real political weight, and that is precisely why they have attracted so much scrutiny and suspicion. Some critics dismiss the entire field as a disguised justification for harsher sentencing – the fear being that if criminality is framed as something people are born with, society can be let off the hook for addressing the conditions that produce it. Other critics take the opposite position and argue that scientific evidence on this subject has been unfairly suppressed to protect a preferred, purely social explanation of crime. Some of the most heated arguments have concerned proposed links between rare chromosomal patterns and criminal behavior – claims that were seized upon, then subjected to fierce scientific rebuttal, and are now treated with considerable caution.
The more measured conclusion, and the one most researchers in this field now hold, is this: heredity shapes temperament and intelligence, both of which are genuinely inherited to some degree, but neither temperament nor intelligence determines behavior on its own. They interact with age, sex, family circumstances, school environment, and community – in ways too complex to reduce to a single cause. Genetic predisposition is a real, measurable piece of the puzzle. It is not destiny, and it is not an excuse. Recognizing that some people begin life more vulnerable to antisocial pathways is not an argument for fatalism; if anything, it strengthens the case for intervening early, before a temperamentally difficult, impulsive child grows into a repeat offender who has spent a decade being punished for a problem no one addressed when it was still small.
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