The placebo effect may not be all in your head - it may be in your genes.

That's what one study published in the journal PLoS One suggests. The multinational team of researchers from Greece, the United Kingdom, and the United States, studied this effect on 104 patients with irritable bowel syndrome (IBS), a disorder characterized by cramping, abdominal pain, and changes in bowel movement.

The question of the placebo effect is an important one in science. Researchers have long wondered why some people respond to placebos, while others do not. And, for drug trials, which pit a medication against a placebo in the hopes that the drug will perform better than the dummy, the placebo effect is incredibly integral to understand.

The patients were divided into three groups. One group called the "augmented group" was given acupuncture from a warm, supportive provider; another group called the "limited group" was given the placebo from a cold, distant provider; and the final group called the "wait list" given no treatment. The "wait list" group allowed researchers to compare the placebo group against people who had received no treatment at all.

In addition to the acupuncture, all patients gave blood samples so that researchers could analyze them for the catechol-O-methyltransferase, or the COMT gene. This gene was singled out for study because previous research had linked it with dopamine response. Kathryn Hall, one of the study authors from Beth Israel Deaconness Medical Center and Harvard Medical Center, said to the BBC that "there has been increasing evidence that the neurotransmitter dopamine is activated when people anticipate and respond to placebos".

Patients with the COMT gene with two copies of the methionine allele (nicknamed met/met) were found to be more likely to respond to the placebo, compared with those in the wait list group. Patients with two copies of the valine allele (val/val) were found least likely to respond to the placebo. Interestingly, people with the met/met variation were also more likely to respond to the acupuncture treatment with additional provider care, which underlines the need for doctor-patient interaction.

"People with met/met look for evidence to confirm their first impression or intuition that something will work versus people with val/val, who if the evidence switches they'll switch more easily," said Ted Kaptchuk, an author of the study and also from Harvard Medical Center and Beth Israel Deaconness Medical Center, to Businesweek. He also said that, as people with the met/met allele begin to recover, "they start seeing more ways of getting better."

Interestingly, previous studies into the subject had found that serotonin, not dopamine, played the largest role in the placebo effect.

The study will have the most practical applications for research that involves participants reporting their own pain or depression. The placebo effect's role in tumor shrinkage, for example, is less understood.