Why do fitness device manufacturers claim you need to take 10,000 steps every day? Do you also really need to drink eight glasses of water daily? The scientific basis for popular health claims is often thin. A piece in the New York Times, note, for example, that the idea of 10,000 steps was based more on marketing—that was the name of an early pedometer—than on science. Data suggest clear benefits of moderate exercise—perhaps 7,000 steps or so, but not necessarily more.
Often, popular wisdom turns out to be only kind of true. The emphasis on so many steps is one example. Glasses of water are another. If you let yourself get too thirsty, you may be tempted to reach for soft drinks or sugary coffee drinks, and that’s not good. But a scientific review in 2002 found “no scientific studies” supporting the eight-glass claim for healthy adults in temperate climates. That doesn’t mean it’s wrong, but it does mean we probably shouldn’t worry if we only drink six.
It’s worth doing a little digging into oft-heard health principles before you take them literally. Sometimes you may find a nugget of truth that has been grossly exaggerated, or you discover that a claim stems from outdated and poorly applied evidence. The latter is what happened to a famous and troubling claim about female fertility.
For decades, women have heard disturbing warnings about their “biological clocks”. We’ve been told repeatedly that fertility drops dramatically after age 30, so people who want children either have to move or else freeze their eggs. Embryo freezing is now big business, and discussions about it are common among young professional women. Pregnancy, and age-related anxiety about it, also occurs among transgender and non-binary people.
But, as Jean M. Twenge in 2013 in the atlantic ocean, the claim is based on very sparse data, many of them of questionable quality or relevance. The idea stems largely from a 2004 paper based on records from 1670 to 1830. Many things have changed since then, including medical care and nutrition. In richer countries, people are now generally healthier and likely to be more fertile for longer periods of their lives. Systematic data collected by the National Center for Health Statistics show that fertility rates for women aged 30 and over rose from 1980 to 2002. It’s also worth noting that as infertility treatments became more common and more clinics opened in the 1980s and 1990s, biological clock alarms were sounded by this growing, self-interested industry.
Despite the importance of fertility to so many people—and while infertility treatments are expensive, have only modest success rates, and are not risk-free—the sad fact is that robust studies of age-related infertility are rare. The data we do have tends to show that while fertility declines slightly at older ages, most women continue to be fertile into their 30s, and for many people this is a good time to have children. There is a long-standing cultural tendency to blame infertility on women, but when a couple is infertile, it is equally likely that the cause can be traced to the man. Male fertility also declines with age, but how often do you hear warnings about the male biological clock?*
Like the female clock or the 10,000 steps, many health beliefs have shallow and thin roots. But sometimes the wisdom of the crowd is backed up by facts: for example, most of us need about eight hours of sleep a night. So where does that leave someone trying to make sense of what they hear or read?
Well, for one thing, people should be skeptical of any big claim based on one study. Good science requires building a multi-layered and detailed case, which takes time and is almost never achieved in a single piece of research. The online medical library PubMed.gov allows people to find out whether a subject has been well studied or not. And the National Institutes of Health has a medical consensus program that has published more than 160 statements about various diseases and their treatments. Some of them are actually readable, and none rely solely on data from over a century ago.
*Editor’s Note (2/23/22): This sentence was edited after it was posted. It originally incorrectly stated that male infertility declines with age.
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