Does Having Children Cause Dementia? What To Make Of A New
Study
Elizabeth Bauer Senior Contributor
Earlier this month, a dramatic headline appeared at the New
York Post:
“No kidding: Rearing 3 or more children could make you
literally lose your mind, study says.”
The dramatic statement from the article, three sentences in?
“The scientists found that having three or more kids vs. two
children has a “negative effect on late-life cognition” — the equivalent of
being 6.2 years older.”
Wowza. Score one for the Zero Population Growth folks, I
guess. Surely there are all manner of factors playing into the equation, right?
But it turns out that there is a “there” there, if you pay close attention to
what the study does and does not say, and it’s not about childbearing decisions
but a much bigger picture.
The actual study (with free public access to the PDF), “Does
Childbearing Affect Cognitive Health in Later Life? Evidence From an
Instrumental Variable Approach,“ by Eric Bonsang and Vergard Skirbekk, was
published last month in the journal Demography. It does not intentionally try
to assess the effect of three vs. two kids but that’s a side effect, so to
speak, of the statistical method the study uses. Here’s how it works:
If one were simply to take a look at the number of children
an older adult has and their cognition according to the tests that measure
this, you wouldn’t really know whether one caused the other, in which
direction, or whether something else entirely caused both. In other words,
you’d just have a correlation. It could be possible that some unknown
predisposition to dementia also causes people to have more rather than fewer
children. Hypothetically, you could get around that by randomly assigning the
number of children people have, but that would rightly be considered seriously
unjust, so scholars have developed econometric methods in which they try to
find situations in real life that resemble random experiments, and that’s what
they did here, using the expectation that at least some of the time, couples
who have either two boys or two girls will try for a third child.
It turns out that this is actually true (or at least was
true) and the study authors can demonstrate it. Using a major ongoing survey of
older adults (ages 50+) across Europe, the authors calculated that, as
expected, of those survey participants who had at least two children, 50% of the
time, those children were both of the same sex, and 50% of the time they had
one of each sex. And a couple with two same-sex first- and second-born children
turns out to have been seven percentage points more likely to have gone on to
have more children, than a couple with one of each. Because of this difference,
researchers can look at the effect of having one of each or a matched pair, on
dementia later in life, do some further math around the proportion of people in
this situation, and voila, you’ve got a calculation which, if it passes the
statistical significance tests, is considered to truly show causality: among
those who have three or more children because they missed having a boy (or a
girl), compared to those who would have tried for that desired boy (or girl)
but didn’t need to because they already had one, there is a greater incidence
of dementia.
Specifically, taking the dementia screening scores from this
study, and creating a composite score where the average score is set at 0 and
the standard deviation set to 1, having more than 2 children reduces a woman’s
dementia score by 0.311 and a man’s dementia score by 0.355.
This math is legitimate, though in order to take the final
step in this mathematical method, called an “instrumental variable,” it
requires making an assumption that the difference in dementia risks and
likelihoods for the group of people who have an “extra” child because they
really wanted at least one boy and girl, compared to those who got the desired
mix on the first try, can be generalized to everyone else.
But what explains this result and what do we do with it? The
answer is not “have fewer children.” Instead, what matters is understanding
what the reason is for this effect and here what’s crucial is their result
dividing up the countries in the study into four quarters: in all four regions
of Northern, Western, Eastern, and Southern Europe, parents were more likely to
have three or more children if their first two children were of the same sex,
but only in Northern Europe did the overall analysis produce the statistically
significant result that dementia was more likely as a result. On the same
measure where the average score on a cognitive test was set at 0 and the range
of scores was adjusted so that a -1 was 1 standard deviation lower, here are
the 3+ child effects on scores for the four regions:
Northern Europe: -0.781 (p = .01)
Western Europe: -0.269 (p = .31)
Eastern Europe: -0.211 (p = .48)
Southern Europe: -0.204 (p = .57)
The p values are a statistical method for telling how likely
a result is just by random chance; for all regions except Northern Europe,
there is a strong likelihood that the apparent reduction in cognition is just
due to flukes in collecting the random sample, but for Northern Europe the
effect is strong enough that it’s no longer credible that it is just random.
So why should this effect be seen in Northern Europe in
particular? The method used in this study doesn’t have any answers to offer but
can only suggest potential explanations. Specifically, there are effects which
have the potential to balance each other out, depending on circumstances: the
more children one has, the more likely it is that one has financial stress due
to added costs and reduced family income due to interrupted employment by
mothers; at the same time, though, having more children boosts the degree of
having contact with children which increases social engagement and reduces the
likelihood of dementia in that way. In Northern Europe, the cost of living is
particularly high so that stresses of a large family may likewise be higher,
despite the social insurance support. At the same time, however, survey
participants in Northern Europe with three or more children do not benefit from
increased rates of contact with their children, in the same was as elders in other
regions do, so they experienced a lose-lose set of circumstances — greater
costs in rearing children without even the payoff of more time spent with
family as they age.
It is also striking to consider the historical context when
looking at this number-of-children cutoff, which, again, they didn’t choose for
any reason other than that only with two children is it possible to have “one
of each.” The waves of the survey took place between 2004 and 2015; while the
survey covered adults beginning at age 50, dementia is most likely after age
80, so that the individuals involved were having their families in the 1960s or
thereabouts. This was the point in time at which, in the United States, we were
transitioning from a Baby Boom to a Baby Bust — as the World Bank data below
makes clear — but the various European countries had much lower birth rates all
along. (To be clear, the World Bank data begins in 1960 so the lack of data
before this point is a matter of availability and says nothing about the
significance of pre-1960 changes.)
At the same time, this was the time when women began
entering the workforce in much larger numbers in these countries. Again, per
the World Bank (and by decade in part due to significant gaps in the earlier
years):
Women in Spain and Italy have had much lower labor force
participation rates, in general — does this mean that the effect of an extra
child might not have affected them in the same way as Scandinavian women? And,
of course, the social supports now prevalent across Europe were in their
infancy two generations ago. All in all, these differences suggest that it
would be very interesting to see the same study repeated in the United States,
where the equivalent study, the Health and Retirement Study, has been conducted
for a decade and a half longer, perhaps, indeed, long enough to see whether
there have been changes over time in the association between three children and
dementia, and drill down further into what sorts of circumstances produce this.
So what’s to be learned? Not that children are bad, of course
— but this does serve as an additional piece of evidence that dementia is not
as simple as unsolvable genetic predispositions, but that there is much more to
be learned about the condition.
https://www.forbes.com/sites/ebauer/2022/05/23/does-having-children-cause-dementia-what-to-make-of-a-new-study/?sh=9d117a9bafde
