The true number across the U.S. population likely falls between these two extremes, but while it's often said that 10 percent of fathers are raising someone else's child, this interpolation isn't quite supported by the facts. The best summations of the data figure an overall prevalence of nonpaternity at more like 2 or 3 percent. One analysis from 2008 looked at several dozen studies going back to the 1890s and found an average rate of 3.1 percent, but also hinted that the numbers might be declining over time (possibly in concert with increasing contraceptive use).
Which is all to say that the expanded 23andMe database may include as many as 30,000 customers like Jackie (3 percent of 1 million) who have gone their whole lives without knowing that their father doesn't share their genes. Even now, among the 250,000 people who have already been genotyped by the company, one might expect that 6,000 or 7,000 were unwittingly involved in cases of nonpaternity. Some of these people have sent off their saliva and gotten back a secret that changed their families forever, for better or for worse.
The rise of personal genomics has not created this phenomenon, of course. Nonpaternity results can arise even in the course of routine medical testing. What happens if a doctor sees that a baby's blood type could not have come from its father? (If the baby's is AB and the father's turns up O, the doctor knows that something is amiss.) In the last few decades, the medical establishment has decided that these findings should be concealed, to protect the mother's privacy and avoid unnecessary harm.
Those who seek that information can get it elsewhere. As of 2011, you can buy an over-the-counter, mail-in paternity test in every state. (The kit costs about $30, plus $129 for analysis.) But these customers know exactly what they're getting into. When people sign up for a service such as 23andMe, they may have no idea that a family secret is about to be exposed.
23andMe does take some steps to warn its users of the risks. The top question on the company FAQ is "What unexpected things might I learn?" and the answer mentions that "genetic information can also reveal that someone you thought you were related to is not your biological kin. This happens most frequently in the case of paternity." The terms of service specify that "once you obtain your Genetic Information, the knowledge is irrevocable," and that "you may learn information about yourself that you do not anticipate" and "may provoke strong emotion."
Yet it's also true that the chances of discovering a case of nonpaternity through 23andMe, and the relative significance of that discovery, far outweigh almost every other finding that the service can provide. Much of what the scan can tell you is perfectly trivial. Do you have the genes for blue eyes or red hair? (For a first approximation, try looking in the mirror.) Do you have the genes for tasting bitterness in Brussels sprouts? (Maybe, but who cares?) After Steven Pinker signed up for 23andMe, he wrote in the New York Times Magazine, "For all the narcissistic pleasure that comes from poring over clues to my inner makeup, I soon realized that I was using my knowledge of myself to make sense of the genetic readout, not the other way around."
Other data points from your personal genomic scan will be more suggestive than deterministic. The test might tell you that you're at a somewhat heightened risk for diabetes or arthritis, but it can be hard to know which bullet points are based on solid science, and which are based on single studies with unconvincing correlations. I asked the company's senior research director Joanna Mountain which genome data would have the most real-world significance for customers, and she named four: Major risk factors for Parkinson's disease, Alzheimer's, a form of heart disease called TTR-related cardiac amyloidosis, and breast and ovarian cancer. (The latter are similar to the risk factor discovered by Angelina Jolie. 23andMe reports on three well-studied variants of the BRCA gene, but there are hundreds of others that may be associated with cancer.)
These are serious conditions, and the risks conferred by certain gene variants appear to be severe. With breast cancer, for example, the relevant mutations may increase the risk of getting the disease from about 13 percent to 60 percent. In Parkinson's, the risk goes up from about 1 or 2 percent to 74 percent. The gene for cardiac amyloidosis can increase the risk of heart failure among elderly African-Americans from 15 percent to 38 percent. Having two copies of the Alzheimer's gene might boost your risk of that disease by a factor of 11.
Given the stakes, 23andMe tries to protect its users. To check your status as a carrier for the genes in question, you must confirm that you're prepared to know the truth and understand the consequences. Even so, the actual risk of carrying these genes is very low. Just 0.013 percent of the population carries the relevant mutations predisposing them to breast and ovarian cancer, for example. (Among Ashkenazi Jews, it's 2 or 3 percent.) One or 2 percent of people will turn out to have the major risk factor for Alzheimer's, and the gene for cardiac amyloidosis matters most to African-Americans, among whom the rate is still just a few percent.
So the chances that you're carrying these genes—the risk that you're at a heightened risk for one of these diseases—tops out at 2 or 3 percent, even in the ethnic groups that are most heavily afflicted. That's directly comparable to the risk of nonpaternity, except when it comes to nonpaternity, we're not talking about people who are merely "carriers" of a twisted gene. If your father's not your father, that's the end of the story. It's not a risk factor; it's a fact.