Haematologists Dr Chi Van Dang and Dr Lawrence Gardner have found that tensing the abdominal muscles and straining the way some people do when they defecate, deliver a baby or have sexual intercourse can produce six to 12 hours of transient global amnesia - the inability to form new memories.

Writing in the medical journal The Lancet, the doctors report the cases of two men, aged 72 and 75, whose wives took them to the hospital half an hour or so after sex when the men became seriously confused. In one case, the patient thought he had suffered a stroke.

Instead, according to the doctors "bearing down" - also known in medicine as a Valsalva manoeuvre - had helped to create intense pressure in the brain's blood vessels resulting in temporary lack of blood flow to the central part of the brain. This, in turn, resulted in amnesia.

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Dr Dang said: "Interestingly this form of amnesia results in a complete inability to recall what happened during the period of confusion.

"As with our patients, who could not recall the name of the current US President, a presidential Valsalva manoeuvre during each of his recent escapades may have legally allowed him not to recall specific events and may thereby help maintain international stability during the current transient global economic fluctuation."

The Valsalva manoeuvre is thought to cause global amnesia in one person in 10,000.

In one reported case, the manoeuvre lead to a ruptured blood vessel in a patient's neck.