Inconsistent parenting leads the child to conclude that behaving well doesn't get results

Professor David Smith

Edinburgh University's Professor David Smith said: "Parents who trust their children but are firm and active in supervising them have a lower degree of conflict than parents who try and lay down the law."

The least successful kind of parenting involved arbitrary and inconsistent attempts to control children with threats that were not carried out, he added.

"Inconsistent parenting leads the child to conclude that behaving well doesn't get results," he added.

More than half the 12 and 13-year-olds questioned for the Edinburgh Study, backed by the Economic and Social Research Council, admitted to two or more "delinquent acts" in the previous year.

They were more likely to have used illegal drugs or to have smoked or drunk alcohol, with 8% saying they had used substances such as cannabis, glue, gas or speed (amphetamine).

Children who had been bullied were more likely to commit offences

About half had been victims of bullying, harassment by adults or crime such as theft, robbery, assault, threats or attacks with a weapon.

"Those who have been victimised and also those who offend tend to have slightly lower self-esteem than others," said Prof Smith.

The researchers measured three personality traits - "impulsivity", or the inability to control immediate impulses, "alienation", which was the belief that the world was against the individual and "self-esteem".

Prof Smith said impulsivity and alienation were strongly linked to both offending and victimisation, helping to explain the relation between the two.

'Delinquency spurt'

There was a surprise on the gender front, according to the researchers.

While delinquency among boys was twice as common, it actually increased more among girls between 12 and 13.

He said: "We expect the boys and girls to draw further apart later, but in the early teenage period girls develop earlier than boys and one consequence seems to be an earlier 'delinquency spurt' along with an earlier 'growth spurt'.

"In particular, there was a rapid increase in smoking among girls so that by age 13 a higher proportion of girls than boys were smokers."