You Can Learn a Lot About Yourself From a DNA Test. Here's What Your Genes Cannot Tell You

“Have you found an article of clothing with a suspicious stain?” asks the website of one Florida-based company called All About Truth DNA Services, which informs readers that “aprrpoximately [sic] 60% of husbands and 40% of wives will have an affair at some point,” and recommends consumers wait for their “suspicious item” to dry and then send it in for testing. Also accepted: cigarette butts, toothpicks, hair.

The landscape of the consumer genomics market now would have been barely recognizable a decade ago. One study by scholar Andelka Phillips, then at the University of Oxford, found that as of January 2016, at least 246 genetics testing companies across the globe were selling their wares directly to customers online. Not all DNA testing companies offer services related to predicting ethnicity and finding relatives; indeed, the spectrum of services they offer is dizzying, and their usefulness and accuracy sometimes dubious. They range from the paternity tests you can pick up at Walgreens to tests that look specifically for African or Native American ancestry to others promising DNA-based matchmaking services. Phillips’s survey placed...