82 Scientific American, March 2022
Naomi Oreskes is a professor of the history of science
at Harvard University. She is author of Why Trust Science?
(Princeton University Press, 2019) and co-author
of Discerning Experts (University of Chicago, 2019).OBSERVATORY
KEEPING AN EYE ON SCIENCE
Illustration by Jay BendtWhy do fitness device makers 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, for example, notes
that the idea of 10,000 steps was based more on marketing—it
was the name of an early pedometer—than science. Data point
to clear benefits from moderate exercise—perhaps 7,000 steps
or so but not necessarily more.
Often popular wisdom turns out to be only sort of true. The
emphasis on so many steps is one instance. Glasses of water is
another. If you let yourself get too thirsty, you may be tempted to
reach for sodas or sugary coffee drinks, and that’s not good. But
a scientific review in 2002 found “no scientific studies” that sup-
port the eight-glass claim for healthy adults in a temperate cli-
mate. That doesn’t mean it’s wrong, but it does mean we proba-
bly shouldn’t worry if we drink only six.
It’s worthwhile to dig a bit into often heard health maxims
before accepting them literally. You can sometimes find a nugget
of truth that has become seriously exaggerated, or you discover
that a claim stems from outdated and poorly applied evidence.
The latter is what happened with a famous and specious claim
about female fertility.
For decades women have heard distressing warnings about
their “biological clocks.” We have been repeatedly told that fer-
tility drops dramatically after age 30, so people who want chil-
dren either need to get moving or else freeze their eggs. Embryo
freezing is now a big business, and discussions of it are common-
place among young professional women. Pregnancy, and age-
linked anxiety about it, also occurs among transgender and non-
binary people.
But, as Jean M. Twenge reported in 2013 in the Atlantic, the
claim is based on very sparse data, much of them of dubious qual-
ity or relevance. The notion stems largely from a 2004 paper based
on records from 1670 through 1830. Many things have changed
since then, including medical care and nutrition. In wealthier
nations, people are now healthier overall and likely to be more fer-
tile for longer periods of their lives. Systematic data collected by
the National Center for Health Statistics demonstrate that from
1980 to 2002 fertility rates for women aged 30 and older were going
up. It is also worth noting that when infertility treatments started
to become more common and more clinics began opening in the
1980s and 1990s, alarms over biological clocks were being sound-
ed by this growing industry with a self-interest in the matter.
Despite the importance of childbearing to so many people—
and although infertility treatments are costly, have only modest
success rates and are not risk-free—the sad fact is that robust stud-
ies of age-dependent infertility are scant. The data we do have
tend to show that although fertility does decline slightly at older
ages, most women continue to be fertile well into their 30s, and
for many people that is a good time to have children. There’s 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 infertility 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 flimsy roots. But sometimes the wisdom of the
crowd is supported by facts: most of us do need around eight
hours of sleep a night, for instance. So where does this leave some-
one trying to make sense of what they hear or read?
Well, for one thing, people should be skeptical of any large
claim based on one study. Good science requires building a mul-
tifaceted and detailed case, which takes time and is almost nev-
er achieved in a single piece of research. The online medical
library PubMed.gov enables people to find out if a subject is well
studied or not. And the National Institutes of Health has a med-
ical consensus program that has published more than 160 state-
ments on various diseases and their treatments. Some of them
are actually readable, and none relies solely on data from more
than a century ago.JOIN THE CONVERSATION ONLINE
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or send a letter to the editor: [email protected]Healthy
Skepticism
Popular notions, such as a woman’s
fertility dropping at age 30, are overblown
By Naomi Oreskes