SEXUAL intercourse before a presentation or speech keeps stress at bay and calms the nerves, scientists have found.

The discovery could be the secret behind great raconteurs and after-dinner speakers.

Having sex with a partner appears to improve a person's ability to cope with stress for up to a week, the research shows. But it has to be full sexual intercourse for the effect to work.

Psychologist Stuart Brody, from Paisley University, compared the impact of different kinds of sexual activity on stress.

For a fortnight, 24 women and 22 men were asked to keep diaries of how often they engaged in full penile-vaginal intercourse (PVI), masturbation or non-penetrative sexual activity with a partner.

Later, the volunteers were given a stress test which involved public speaking and doing mental arithmetic sums out loud.

Those from the PVI group were least stressed, and their blood pressure returned to normal faster than it did for the others. Participants who had abstained from sex had the highest blood pressure response to stress.

Mr Brody's team tested for psychological traits, such as neuroticism and anxiety in the volunteers, as well as work stress and partnership satisfaction.

Even taking these factors into account, differences in sexual behaviour provided the best explanation for the range of stress responses.

"The effects are not attributable simply to the short-term relief afforded by orgasm, but rather, endure for at least a week, " Mr Brody told New Scientist magazine.

He believes that release of the "pair-bonding" hormone oxytocin between partners might account for the calming effect.