What’s the difference between a man and a woman? The question seems too silly to be science: Its answer is so obvious that every stand-up comic has a different way of saying it. (Even the gender jokes break down by gender: Male comics say women go to the bathroom in packs. Female comics say men wouldn’t understand what they talk about in there anyway.)

But the difference is only obvious most of the time. In some unusual cases, resulting from sex-change operations or medical conditions, the usual indicators of male and female can contradict each other in the same body. The best-known recent example is South African runner Caster Semenya, who has been put through “gender verification” amid suspicion about her muscular physique and low voice.

But the same confusion has cropped up in legal battles over who can be married to whom, and when the “M” on a driver’s license can be changed to “F”. It can also intrude painfully into the lives of ordinary people, when medical diagnoses reveal that their hormones, chromosomes or anatomy don’t sit entirely on one side of the line.

These cases have left judges, doctors and athletics officials—those tasked with drawing a bright line between the sexes—struggling to find a reliable gender test, some trait that divides ALL men from ALL women. But scientists say they don’t have one yet. “I think most people think of it in binary terms—that is, you’re either one or the other,” male or female, said Myron Genel, a professor emeritus of pediatrics at Yale University. “In reality, it’s more of a continuum.”

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