The theory is that firstborns are more likely to have a competitive personality - and thus to deal badly with stress.

The family context frequently orients them along a perfectionist path, giving them a determined, competitive, winning and aggressive attitude

Dr Maurizio Ferratini

They found 46.7% of the CHD patients were firstborn - almost twice the proportion in the general population.

The researchers did not set out to identify the reasons why firstborns are seemingly at greater risk.

Certainly, there were no significant differences between firstborns and their sisters and brothers in terms of family history of CHD, high blood pressure or diabetes.

However, they believe that personality may be a significant factor.

Hard-driven

Dr Ferratini said firstborns were more likely to have a type A personality, characterised as being hard-driven, easy to anger and highly motivated.

People with this type of personality are thought to be more prone to heart disease.

He said: "The family context frequently orients them along a perfectionist path, giving them a determined, competitive, winning and aggressive attitude."

Dr Ferratini said that more research was required to determine just how much more at risk firstborns were.

But he suggested it was a factor that doctors should take note of when assessing a person's risk of heart disease.

A spokesman for the British Heart Foundation told BBC News Online there was little evidence to suggest that firstborn children were at greater risk of heart disease.

He said: "It is possible that people with a type A personality are more likely to take up smoking, drinking and eating unhealthily - all factors that are proven to increase the risk of heart disease."

The research was presented at a meeting of the American Heart Association.