The manipulative con-man. The guy who lies to your face, even when he doesn’t have to. The child who tortures animals. The cold-blooded killer. Psychopaths are characterised by an absence of empathy and poor impulse control, with a total lack of conscience. About 1% of the total population can be defined as psychopaths, according to a detailed psychological profile checklist. They tend to be egocentric, callous, manipulative, deceptive, superficial, irresponsible and parasitic, even predatory. The majority of psychopaths are not violent and many do very well in jobs where their personality traits are advantageous and their social tendencies tolerated. However, some have a predisposition to calculated, “instrumental” violence; violence that is cold-blooded, planned and goal-directed. Psychopaths are vastly over-represented among criminals; it is estimated they make up about 20% of the inmates of most prisons. They commit over half of all violent crimes and are 3-4 times more likely to re-offend. They are almost entirely refractory to rehabilitation. These are not nice people.
So how did they get that way? Is it an innate biological condition, a result of social experience, or an interaction between these factors? Longitudinal studies have shown that the personality traits associated with psychopathy are highly stable over time. Early warning signs including “callous-unemotional traits” and antisocial behaviour can be identified in childhood and are highly predictive of future psychopathy. Large-scale twin studies have shown that these traits are highly heritable – identical twins, who share 100% of their genes, are much more similar to each other in this trait than fraternal twins, who share only 50% of their genes. In one study, over 80% of the variation in the callous-unemotional trait across the population was due to genetic differences. In contrast, the effect of a shared family environment was almost nil. Psychopathy seems to be a lifelong trait, or combination of traits, which are heavily influenced by genes and hardly at all by social upbringing.
The two defining characteristics of psychopaths, blunted emotional response to negative stimuli, coupled with poor impulse control, can both be measured in psychological and neuroimaging experiments. Several studies have found decreased responsiveness of the amygdala to fearful or other negative stimuli in psychopaths. They do not seem to process heavily loaded emotional words, like “rape”, for example, any differently from how they process neutral words, like “table”. This lack of response to negative stimuli can be measured in other ways, such as the failure to induce a galvanic skin response (heightened skin conduction due to sweating) when faced with an impending electrical shock. Psychopaths have also been found to underactivate limbic (emotional) regions of the brain during aversive learning, correlating with an insensitivity to negative reinforcement. The psychopath really just doesn’t care. In this, psychopaths differ from many people who are prone to sudden, impulsive violence, in that those people tend to have a hypersensitive negative emotional response to what would otherwise be relatively innocuous stimuli.
What these two groups have in common is poor impulse control. This faculty relies on the part of the brain called the prefrontal cortex, most particularly the orbitofrontal cortex. It is known that lesions to this part of the brain impair planning, prediction of consequences, and inhibition of socially unacceptable behaviour – the cognitive mechanisms of “free won’t”, rather than free will. This brain region is also normally activated by aversive learning, and this activation is also reduced in psychopaths. In addition, both the prefrontal cortex and the amygdala show substantial average reductions in size in psychopaths, suggesting a structural difference in their brains.
These findings have now been united by a recent study that directly analysed connectivity between these two regions. Using diffusion tensor imaging (see post of August 31st 2009), Craig and colleagues found that a measure of the integrity of the axonal tract connecting these two regions, called the uncinate fasciculus, was significantly reduced in psychopaths. Importantly, connectivity of these regions to other parts of the brain was normal. These data thus suggest a specific disruption of the network connecting orbitofrontal cortex and amygdala in psychopaths, the degree of which correlated strongly with the subjects’ scores on the psychopathy checklist.
All of these findings are pointing to a picture of psychopathy as an innate, genetically driven difference in connectivity between parts of the brain that normally drive empathy, conscience and impulse control. Not a fault necessarily, and not something that could be classified as a disease or that is always a disadvantage. At a certain frequency in the population, the traits of psychopathy may be highly advantageous to the individual.
This conclusion has serious ethical and legal implications. Could a psychopath mount a legal defense by saying “my brain made me do it”? Or my “genes made me do it”? Is this any different from saying my rotten childhood made me do it? Psychopaths know right from wrong – they just don’t care. That is what society calls “bad”, not “mad”. But if they are constitutionally incapable of caring, can they really be blamed for it? On the other hand, if violent psychopaths are a continuing danger to society and completely refractory to rehabilitation, what is to be done with them? Perhaps, as has been proposed in the UK, people with the extreme psychopathic personality profile (or maybe in the near future even a specific genetic profile?) should be monitored or segregated even before they commit a crime.
While it is crucial that these debates are informed by good science, these issues have no clear-cut answers. They will be resolved on a pragmatic basis, weighing the behaviour that society is willing to tolerate versus the rights of the individual, whatever their brains look like, to define their own moral standards.
Craig, M., Catani, M., Deeley, Q., Latham, R., Daly, E., Kanaan, R., Picchioni, M., McGuire, P., Fahy, T., & Murphy, D. (2009). Altered connections on the road to psychopathy Molecular Psychiatry, 14 (10), 946-953 DOI: 10.1038/mp.2009.40