Scientists have long known that people perceive scents differently. But emerging evidence from several large-scale studies shows that the variation is larger than previously known. It turns out that people differ in how they perceive many if not all odors, and most of us have at least one scent we cannot detect at all. “Everybody’s olfactory world is a unique, private world,” says Andreas Keller, a geneticist at the Rockefeller University.

Over the course of evolution, partly because humans grew more reliant on vision and smell became relatively less important, the genes encoding our 400 or so olfactory receptors began to accumulate mutations. Once a gene has accumulated enough mutations, it becomes a “pseudogene,” notes geneticist Doron Lancet of Israel’s Weizmann Institute of Science, meaning it no longer encodes a functioning receptor. Different people have different combinations of pseudogenes, however. “You end up with a bar code situation, whereby each individual has a slightly different bar code,” he says.

That genetic variability seems to translate into behavioral variability. When  Keller and his colleagues asked 500 people to rate a panel of 66 odors for intensity and pleasantness, they gave the full range of responses—from weak to intense and from pleasant to unpleasant. In an ongoing study at the University of Dresden, Thomas Hummel and his associates have tested 1,500 young adults on a panel of 20 odors and found specific insensitivities to all but one—citralva, which has a citrus smell. Based on these findings, Keller suspects that each person has an olfactory blind spot.

These studies have wider implications than smell, Lancet says. Because several genes contribute to the detection of most odors, understanding the genetics of olfaction and the way mutations spread in a population is yielding insights into the mechanisms of poly­genic diseases such as coronary heart disease and diabetes.