Psychology of Crime: Why Do People Become Criminals?
Psychological theories of crime
Many people have their own theories on what makes a criminal. Some of these theories are based on first hand knowledge or experience, some unfortunately may be based on racism or prejudice, and some on scientifically investigated studies.
And there are several psychological theories of crime, most of which have been shown to have a sound scientific basis. However, it is widely accepted that the reasons for crime are seldom one cause or the other, but rather a combination of some.
Biological Theories of Crime
These include genetics, hormones, brain chemistry (neurotransmitters) and brain structure and anatomy.
Because statistically more males commit crimes than females, it was proposed that this must be because of the genetic make-up of males. However, this theory has been largely discredited
Twin studies and crime
But studies with twins have shown that identical twins are more likely to share criminal tendencies than non-identical (or fraternal) twins. This was the case even when identical twins were separated at birth, so environment or upbringing would not necessarily have been a factor .
Even so, some psychologists still believe that this is not conclusive evidence of a genetic link.
Serotonin is a neurotransmitter in the brain that affects mood, which in turn can affect criminal behavior. Testosterone, the male hormone, is linked to levels of aggression. Omega 3 has been shown to lower levels of aggression, and poor nutrition before the age of 3 has also been linked to higher levels of aggression. All of these come under the heading of Brain Chemistry and all have a link to criminal behavior.
Brain Structure and Anatomy:
The part of the brain associated with or emotions is called the Amygdala (am-ig-d-la). It is believed that damage to the Amygdala can have an effect of criminal behavior.  This may be because the person concerned would have a limited fear and conditioning response, thus fear of punishment would not deter them from committing a crime.
The Hippocampus is where we store our memories. Damage to this area could mean we do not remember being punished for our crimes, and so would commit them again and again.
The Frontal Cortex, as the name suggests, is to the front of our brain and would also appear to be involved, among other functions, with our self control-as one famous case-study showed:
The most famous case of brain damage causing a change in self-control is one of a man called Phineas Gage. In 1848, Phineas was a mild-mannered and conscientious railroad worker foreman in Vermont, U.S. He was overseeing the laying of explosives one fateful day. It was the practice to lay sand over the explosives in a hole and then to tap it down with a tamping iron. Phineas was using the tamping iron, which was 3’8” long and 1.5” in diameter, when a spark ignited the explosive and sent the tamping iron straight through his left cheek and out through the frontal cortex, landing several feet behind him. Incredibly, Phineas not only survived, but walked to the cart which was to transport him to a doctor.
"No Longer Gage"
While Phineas later appeared to have made a full recovery, those who knew him before the accident said that he was “No longer Gage” No longer mild mannered and conscientious, he became verbally aggressive and abusive, unreliable in his work and impatient and impulsive to the extent that the railroad company could no longer employ him.
Was it the brain damage that caused the change?
It appeared that the damage to the frontal cortex caused the change in Phineas. However, it must also be remembered that brain damage also has the potential to cause depression, and that there was also a possibility that Phineas would have suffered from Post Traumatic Stress, either of which could also cause changes in his personal disposition.
Socialisation theories of crime
These include Learning Theories such as:
- Classical Conditioning-the famous example being Pavlov’s Dogs, in which Pavlov trained the dogs to salivate at the sound of a bell.
- Operant Conditioning-The Skinner Box, developed by B.F Skinner (who else?) in which he trained rats to press (or ‘operate’) levers in order to get to their food.
- Observational Learning-“Monkey see-Monkey do”
But humans are not dogs, rats or monkeys. However, it would seem that we do learn by similar methods. If a child is surrounded by crime, either within the family or the community, they are likely to learn criminal behaviour by any or all of the above methods.
Routine Activity Theory
This can tie-in somewhat with Learning Activity; for example, if a child learns that stealing is one way to get what they want, they will do it again. All they need is for three elements to be in place:
1.Motivation: They want something
2.Suitable Target: They see what they want
3.Absence of Guardians: And there’s no one about
They get away with it, and do it again and again, until it becomes routine .
This is probably one of the best known psychological theories of crime.
A person really wants something, such as material goods, a better lifestyle or even an education, but they can see no possible way of ever achieving it now or in the future. This understandably causes dissatisfaction, perhaps even resentment against the people who do have what they want.
But then they see there is a way to achieve their desires through stealing, drug dealing or other criminal behaviour .
A Marxist theory, which says that the Criminal Justice System is seen as being developed by the dominant classes to the sole advantage of the dominant classes, causing resentment and rebellion. 
Social Construction Theory of Crime
Each society has their own view of what is and is not a crime: For example, in Saudi Arabia, public displays of affection are illegal.
Circumstance can also change whether certain behavior is a crime or not. For example, a Police car or an Ambulance may break the speed limit without suffering a penalty.
Society’s view of crime can also change with time; for example, Prohibition, Homosexuality, and more recently, Cyber crimes.
Just some of the theories
These are just some of the more well-known psychological theories of crime.
If you want to learn more, I suggest you consult any good textbook on forensic or criminal psychology.
Meanwhile, take a look at the video below for Rational Choice theory..
 Howit, D., (2009), Introduction to forensic and criminal psychology (3rd ed) Harlow, UK, Pearson Education.
 Viding, E., Blair, R. R., Moffitt, T. E., & Plomin, R. (2005). Evidence for substantial genetic risk for psychopathy in 7-year-olds. Journal Of Child Psychology & Psychiatry, 46(6), 592-597. doi:10.1111/j.1469-7610.2004.00393.x
 Raine, A. (2008). From Genes to Brain to Antisocial Behavior. Current Directions In Psychological Science (Wiley-Blackwell), 17(5), 323-328. doi:10.1111/j.1467-8721.2008.00599.x
Clarke, R. V., & Felson, M. (1993). Routine activity and rational choice. Piscataway, NJ US: Transaction Publishers
 Agnew, R. (1993). Why do they do it? An examination of the intervening mechanisms between "social control" variables and delinquency. Journal Of Research In Crime & Delinquency, 30(3), 245-266.
 Bonger, W. (1916) Crime and Economic Conditions. Boston. Little Brown.
Questions & Answers
What factors create a criminal?
There is seldom just one factor, rather a combination.Helpful 6
What causes brain damage?
There is not just one cause of brain damage. It can be caused by an accident, a difficult birth, a birth defect, a disease or other causes.Helpful 4
Why is it that siblings can be in the same environment and one shows a high level of criminality and the others do not?
As mentioned in the article, there are many theories of why someone becomes a criminal, but it is generally agreed that it is a combination of genes, events, conditioning, and personality. It has been shown in studies with twins that identical twins are more likely to share criminal tendencies than non-identical (or fraternal) twins. This might indicate a genetic link, as identical twins have more genetic similarities, but also that personality is involved, in the case of the fraternal twins. There also is a theory that where siblings come in the family can have an effect on their behavior. For example, middle children often show more challenging behaviors and it's theorized that it is because they are trying to get attention. I'm not a forensic psychologist, so that is about the limit of my knowledge.Helpful 3