Your HLA profile is very likely to be different to everyone else you meet – though some people, like your close relatives, will be more similar to you than others. From a genetic point of view, it is an advantage to have a child with someone who has a dissimilar HLA profile. “If you have a partner who is genetically dissimilar in BO and immune profile, then your children will have a better resistance to pathogens,” says Sorokowska. These women put the T-shirts worn by men with the most dissimilar HLA profile first and last the most similar. So they were able to identify the men, and preferred the men, with the best match in terms of immune system genetics. They didn't know that was what they were doing, of course – it was subconscious. The specific mechanism that causes HLA-dissimilarity to result in a better-smelling BO is not known, says Sorokowska. "But it is thought that HLA results in the production of certain substances that are digested by our skin bacteria that produce a certain odour."