By Caroline Ryan
BBC News Online health staff
Many people tell the odd white lie - taking a day off "sick" or halving the amount they spend on a shopping trip.
Good relationships with adults as children are 'crucial'
But most feel a little bit guilty about the deception.
Scientists have now found that twinge of conscience can be seen in increased activity in the brain.
In short, it takes more effort to lie than to tell the truth.
But people with psychopathic tendencies find lying as easy as telling the truth.
The reason is that when children develop the ability to deceive - around the age of three of four - they also develop the ability to empathise.
But researchers say people with aggressive and antisocial personality disorders do not develop this ability, and therefore they have no moral compass.
They say traumatic experiences and a lack of contact with understanding adults could be to blame.
'Deception is OK'
The reasons why some people become psychopathic will be discussed at a conference in Sheffield entitled "Psychopathy and the Problem of Evil".
Dr Sean Spence of the University of Sheffield, who is chairing the conference, has found parts of the frontal lobe area of the brain are more active when someone was lying than when they were telling the truth.
He will prevent evidence confirming earlier studies which showed the neurological activity involved in lying.
Dr Spence said: "When we're lying, there is a moral part of us that doesn't wish to manipulate others or take advantage of them.
"In psychopaths, there is no activity in that area of the brain, and deception is OK to them.
"They don't have any qualms about doing it."
He said a lack of adults displaying empathy towards them as children meant psychopaths could not learn from example, and developed the aggressive antisocial personality disorder.
"If they have experienced gross sexual abuse of severe physical violence, they may never have been in contact with the feeling of empathy."
Dr Spence said 'good parenting' was a crucial part of preventing these tendencies developing.
"Even if people have had an experience such as sexual abuse, if they have had at least one good relationship with an adult figure, they don't become delinquent."
Diet and exercise
Childhood diets could also influence whether people develop psychopathic tendencies, experts say.
The conference will hear from Professor Adrian Raine, a psychologist from the University of California.
He gave a group of three-year-olds from Mauritius a programme of an enriched diet, exercise and cognitive stimulation - being read to and involved in conversation.
By the age of 11, they showed increased activity on brain scan readings, and by 23, they were 64% less likely than a group of children who had not been on the programme to have criminal records.
Professor Raine said: "This is not a silver bullet to solving crime and violence, but I think it's certainly one of the ingredients.
"The take-home point is that the seeds of crime are sown early in life."
Dr Spence added that alcohol or drug abuse could also cause damage to the brain and cause psychopathic behaviour.
But he said that even people who had never before shown any signs of psychopathic behaviour could behave very cruelly in extreme situations.
"In Rwanda, around 800,000 people were killed in 100 days.
"Most people doing the killing had been 'normal' before - it was something in their environment that changed."