Why does health in early life matter?

Children are less healthy at birth when they are born to low income families, and their health in early life echoes into adulthood determining the chances of success and independence decades later.

It is well known that humble beginnings are a handicap, argues Janet Currie of Princeton University, but careful analysis is needed to understand the mechanisms and to appreciate the extent to which health status at birth causes longer run outcomes associated with success in adulthood.

In a presentation made to the annual meeting of the Canadian Economics Association, held over the weekend at the University of Calgary, she suggested that extensive research by labour economists documents the causal impact of low birth weight and has policy implications for not only the poor, but also the rich.

If low birth weight causes low socio-economic status in adulthood, rather than just being associated with it, then there is a strong case for public policy to be concerned about the well-being of low income women in their childbearing years.

But this causal relationship also raises cautions about in vitro fertilization, generally used more by the relatively well-to-do, and also more likely to lead to infants with lower birth weights.

In response to a question after her presentation she said, “from the perspective of child health it is very dubious to be subsidizing in vitro fertilization. It will lead to much lower child health.”

In 2010 Quebec became the first jurisdiction in North America to support in vitro fertilization through the public purse, in part for pro-natalist reasons. But the procedure has a tendency to lead to multiple births and, even when it doesn’t, to lower birth weight. This, in part, is the basis for Dr. Currie’s comments.

Professor Currie, who was born in Kingston Ontario, offered an overview of an extensive academic literature examining the relationship between the socio-economic status of mothers and the birth weight of their children, showing that:

1. there are large inequalities in health at birth with women of lower socio-economic status having children with lower birth weights;

2. lower birth weight predicts important outcomes like schooling attainment, labour market engagement, and reliance on social assistance in adulthood;

2. the transmission of poverty across the generations should not be considered as genetic, even though girls with low birth weights grow up to in turn have low birth weight children;

4. there may be remedial policies that can reverse this intergenerational cycle of low health and socio-economic status.

These patterns exist across many of the rich countries, whether or not they have public health care. Poor children are more susceptible to risks that adversely influence their health, even though they may have more access to hospitals and health care in some countries than in others.

Dr. Currie is particularly concerned with dispelling the idea that these risks are genetic.

“To say something at birth is genetic is code for saying we can’t do anything about it.” She suggests that biologists increasingly feel that environmental factors determine how genes are expressed, and that therefore the distinction between nature and nurture is a false dichotomy.

She points out that maternal education plays an important role in determining birth weight by citing the findings from one of her own research papers called: “Mother’s education and the intergenerational transmission of human capital“.

This is normally a challenging assertion to make because causation cannot easily be inferred from association: maternal education and birth outcomes may be determined jointly by some other factor like genetic endowments.

Her research finds that young American women who at the age of 17 happened to live in a county in which a new university campus opened, obtained more education than women of similar socio-economic background who lived further away.

The variation in education outcomes due to the pure chance of living in the right place, and therefore having easier access to higher education, ends up influencing maternal behaviour and improves birth weight outcomes.

For the most part this channel works through the impact more education has on lowering smoking, and thereby improving the in utero environment.

While these sorts of “experiments,” that happen because some policy changes influence similar people in different ways, offer opportunities to examine the causal impact of interventions, they do not occur as often or in the way that researchers would like.

Even so Currie is confident in going on to argue that birth weight is not simply associated with longer-term outcomes, but also causes them.

The literature she cites makes extensive use of another type of “experiment” based upon differences between siblings, children sharing similar environments during the early years but who differ in their birth weight.

A whole host of studies—some of the most convincing being done using administrative data from Manitoba that links health care, schooling, and social assistance data files—have offered important insights.

Differences between siblings in education, the use of social assistance at age 17, height, IQ, full-time earnings, and even the intergenerational transmission of low birth weight, have all been shown to be determined by differences in birth weight.

And these studies also show that the impact of birth weight is much smaller when children are born in higher income families, suggesting the effect is not immutable. In particular, Professor Currie cites research by Mark Stabile of the University of Toronto and Kevin Milligan of the University of British Columbia showing that higher child benefits improve child outcomes.