Dr. Loren Martin and his colleagues were actually investigating another question when they discovered this surprising result. They were measuring how multiple sources of pain changed pain perception.
In experiments, in mice they used a heat probe that created an mild level of heat on the mouse's feet. Then they gave the mice a dose of vinegar to upset their stomachs. The mice, unsurprisingly, didn't like it.
The suprise came when they they repeated the experiment. The male mice showed more stress when brought back to the location of the experiment, and had stronger responses to the heat stimuli - they were more sensitivity to the pain. The female mice showed no extra stress or sensitivity.
Researchers shone a mild heat probe at the mice's feet to induce a slight pain in the experiment. (Sana Khan)
They then ran a similar experiment on humans.
They used the same combination of stimuli - heat on the forearm, and an uncomfortably tight blood pressure cuff over the bicep. They left the cuff on for about 20 minutes and had the participants do arm exercises to increase the pain.
The participants came back the next day and were sent to the same environment where they wore the blood pressure cuff to repeat the heat probe experiment. This time, the men reported feeling more pain from the heat probe, while nothing changed for the women. It was the same sex difference that they'd seen in the mice.