Thursday's Global Handwashing Day aims to improve hand hygiene by children to prevent diarrheal disease. (Mary Altaffer/Associated Press)

Both men and women are more likely to wash their hands properly after using the toilet if they feel they're being watched, British researchers say.

"Handwashing with soap has been ranked the most cost-effective intervention for the worldwide control of disease," the study's authors wrote in the October issue of the American Journal of Public Health.

"It could save more than a million lives a year from diarrheal diseases and prevent respiratory infections — the biggest causes of child mortality in developing countries."

Handwashing is also promoted as a cheap way to prevent the spread of respiratory and hospital-acquired infections such as C. difficile.

But only 32 per cent of men and 64 per cent of women washed their hands with soap, Robert Aunger of the London School of Hygiene & Tropical Medicine and his colleagues found.

The researchers tested a variety of messages that were flashed on LED screens at the entrance of the toilets to see how behaviour changed.

The message that elicited the strongest positive response was: "Is the person next to you washing with soap?" — which suggests people respond more when they think others are watching.

Men and women also reacted differently to the messages, the researchers found.

Men tended to respond best to messages that invoked disgust, such as "Soap it off or eat it later," or "Don't take the loo with you — wash with soap," increasing soap use by 9.8 per cent.

Women tended to respond to simple reminders like "Water doesn't kill germs, soap does," increasing soap use by 9.4 per cent compared with a control.

"Our data show that unobtrusive observation of behaviour in a natural setting can help identify the most effective interventions for changing behaviours of public health importance. The gender differences we found suggest that public health interventions should target men and women differently," the researchers concluded.

Thursday is Global Handwashing Day, which aims to improve handwashing practices, particularly among children.