Tuesday, October 19, 1999 Published at 12:57 GMT 13:57 UK
Morals on the brain
Damage to the part of the brain which learns moral and social rules could cause children to grow up into irresponsible adults and even criminals, new research suggests.
The scientists warn that it is too early to draw firm conclusions and that their work does not mean that all antisocial behaviour can be blamed on damage to this area.
But an intriguing finding is that patients with these problems do not learn lessons from being punished after misbehaving. This could call into question the effectiveness of criminal penalties when applied to this group.
The affected brain area is called the prefrontal cortex and has long been known to affect social behaviour, without affecting general intelligence. Its function is illustrated by the classic case of railway worker Phineas Gage. In an 1848 explosion, a metal rod was driven through his prefrontal cortex.
Before the accident he had been industrious, dependable and well-liked, but afterwards he became a drifter who was profane, unreliable, impulsive and inconsiderate to his family.
The new work by neuroscientists from the University of Iowa investigated two individuals who had suffered damage to the pre-frontal cortex as babies.
One was a girl aged 20 who had been knocked down by a car at 15 months. The other was a man aged 23, who had undergone brain surgery at three months.
Both children recovered well and were nurtured in middle-class families with educated parents. But when they reached adolescence, their behaviour changed dramatically - they lied, became selfish, lazy and disruptive, started fights and stole money.
They also became sexually reckless, becoming parents of children that they then neglected.
In both cases, the brain's normal cognitive functions, such as reading and writing, were unaffected. What was affected was an ability to realise the social consequences of misbehaviour and to carry out moral reasoning.
Learning the rules
The team also noticed a difference between those people brain injured as children and those damaged as adults. The adult patients understood moral and social rules but appeared unable to apply them to their own lives.
Those damaged at an early age seemed unable to learn the rules in the first place, having as adults the moral reasoning skills of 10 year olds. They also were more likely to exhibit psychopathic behaviour like stealing and being violent.
The researchers acknowledged that two is a small number of case studies but noted that it is hard to find documented cases where brain damage has such restricted effects.
The research is published in Nature Neuroscience.