Variations in a single gene may explain why some people can withstand pain that would cause others to crumple.
Pain tolerance varies
The gene in question, which comes in two forms, makes an enzyme that helps control the brain signals involved in pain response.
Everyone carries two copies of the gene, one inherited from each parent.
A study of volunteers showed that those with two copies of one form of the gene felt much more pain than those with two copies of the other.
Examining and detailing the biochemistry of these processes can then lead to more effective treatments
People with one copy of each gene variation had a pain response somewhere between the two.
The gene variants are known as "val" or "met". They make versions of an enzyme called catechol-O-methyl transferase (COMT) which differ in only the tiniest of ways.
Lead researcher Dr Jon-Kar Zubieta, from the University of Michigan in Ann Arbor, said: "Participants who had two copies of the val form withstood quite a bit more pain than others in the study, while at the same time reporting that they felt less pain and fewer pain-related negative
"Our findings and those of other groups underlie the need to think about pain as the result of complex interfaces between injury and our own capacity to regulate its severity and significance.
"Examining and detailing the biochemistry of these processes can then lead to more effective treatments."
The researchers used brain scans to trace the brain's pain responses in 29 volunteers.
Each was given carefully controlled salt-water injections in their jaw muscles to induce pain.
Subjects rated their pain every 15 seconds during each scan and filled out standardised questionnaires assessing how they felt.
The COMT enzyme breaks down chemicals such as dopamine which transmit messages between brain cells.
Two copies of the "val" gene makes powerful COMT that mops up the chemicals rapidly.
But two copies of the "met" form produce a weaker version of COMT that is not so good at clearing away the chemicals.
It is thought that a build up of chemicals like dopamine suppresses the activity of the brain's natural painkillers - the endogenous opioids.
The study found that volunteers with two copies of the "val"
gene were better able to break down dopamine and suffered less pain.
The research is published in the magazine Science.