(Inside Science) -- Being sick doesn't just make you feel lousy; it also makes you look and smell less likeable, according to a new study. Being wary of sick-looking neighbors may have given our ancestors an adaptive edge, helping them to avoid getting sick themselves.
To test how people perceive sickness in others, the researchers injected 18 subjects with chemicals that trigger an immune response, essentially tricking their bodies into thinking they had a bacterial infection. They took photographs of the subjects' faces before and two hours after the injection, and also collected their body odors by sewing nursing pads into the armpits of their shirts.
Next, they placed a second group of 30 people in fMRI machines that measured brain activity, and showed them pictures of the first group's faces. At the same time, they exposed the fMRI group to either pre-injection "healthy" body odors, post-injection "sick" body odors, or the smell of an unused nursing pad, each odor so faint as to be barely detectible.
As expected, the fMRI group judged photographs as more likeable when they showed healthy, pre-injection faces, and less likeable when they showed people who appeared sick. Compared with the smell of an unused nursing pad, "sick" body odors made faces appear less likeable, while "healthy" body odors had no significant effect.
When people could both smell and see "sick" people at the same time, some parts of their brains lit up with extra intensity, showing more activity than you would get from simply adding up the effects of sight and scent alone. These brain regions included the superior temporal sulcus and the superior temporal gyrus, which are thought to integrate information from different senses, as well as the hippocampus and amygdala, which are involved in memory, emotion and fight-or-flight responses. According to the researchers, the findings suggest that the brain may integrate subtle signals about the smell and sight of sickness into a larger picture of the potential threat.
"Their brains really responded differently to sickness and health," said Christiana Regenbogen, a neuroscientist at the Karolinska Institute in Stockholm and at RWTH Aachen University in Aachen, Germany, and lead author of the study published today in Proceedings of the National Academy of Sciences. "There are very early and weak signs of sickness that we can detect as healthy humans, which could act as an avoidance mechanism to avoid contagion."