A dense, hierarchically connected series of sensitive periods occur in brain and biological development during the first years of life, which mean that developing children are profoundly influenced by the environments in which they grow up, live and learn (4
). Because of this, sensitive periods represent windows of opportunity to improve, or to damage, a child’s future life chances.
In Canada, disparities that emerge early in life in children’s physical, social/emotional and language/cognitive development are largely attributable to systematic differences in the qualities of their early environments. For example, the number of words a child can recognize and express by three years of age varies by more than threefold among otherwise normally developing children. The reason is that receptive and expressive language skills improve in a ‘dose-response’ fashion with the number and variety of words spoken directly to them, and the forms in which language is used, during those three years (5
). It has been shown that by the time they reach school age, the American child who has heard the most language has actually heard approximately 30 million more words than the child who has heard the least (5
). The lesson is that relatively small, day-to-day differences in the child’s environment – in this case, how much they are spoken to each day – can have large cumulative effects over the early childhood years. The same principle of dose-response, in relation to early stimulation, applies to physical and social/emotional development.
Through Canada’s National Longitudinal Survey of Children and Youth, we know that a child’s chances of being vulnerable (ie, being behind where we would like them to be in their physical, social/emotional or language/cognitive development) by the time they reach school age forms a gradient as one goes from the top to the bottom of the spectrum of family income, parental education or the status of parents’ jobs (6
). The nature of the gradient is very important for clinicians and policy-makers alike to understand. The gradient shows that the fraction of vulnerable children gradually increases without a threshold as one goes from the most privileged to the least privileged families. Children in the least privileged families have the greatest chance of being vulnerable, but, from the perspective of society, the largest number (ie, the numerical majority) of vulnerable children is spread more thinly across the (much more numerous) middle class. We have now shown that, in Canada, no specific population can be targeted – by income, ethnicity, immigration, birth weight, gestational age, family psychosocial risk status or adverse neighbourhood circumstances – that will contain a majority of the children who will be developmentally vulnerable by the time they reach school age (8
). Thus, a clear implication of the gradient is that improving the state of early development means finding ways to provide access to strong nurturant environments to all children. This may be achieved using different strategies in different contexts; however, the ultimate goal must be universal access to environments that will minimize vulnerability and support healthy child development. This is what ‘equity from the start’ is all about.
Gradients are also important because, once established, they track forward over the life course. Among the wealthy countries, a gradient (according to parental education) is seen with respect to achieving the literacy and numeracy skills needed to cope in an information society (9
). But, there are important differences among societies. In countries where gradients are the flattest (ie, where outcomes across the socioeconomic spectrum are relatively equitable), the proportion of children who do not achieve adequate literacy and numeracy skills is relatively low (as low as 10%), whereas in countries with steep gradients, it is much higher overall (30% to 40%). In other words, pursuing policies that flatten gradients is good for society.
Thanks to a series of birth cohort studies (10
) from different countries, we now know a great deal about the influence of the early childhood years on health, well-being, learning and behaviour across the life course. By the second decade of life, experiences in early life are associated with the risk of school failure, antisocial behaviour and teen pregnancy. By the third and fourth decades, they are associated with obesity, high blood pressure and depression; by the fifth and sixth decades, with coronary heart disease and diabetes; and by late life, with premature aging and memory loss.
We are currently on the leading edge of a revolution in the science of early child development. We now know that a process called ‘biological embedding’ exists, which has the following characteristics: experience gets ‘under the skin’ and alters human biodevelopment; systematic differences in experience in different social environments lead to different biodevelopmental states; these differences are stable and long term; and they have the capacity to influence health, well-being, learning and behaviour over the life course (11
). We now recognize the remarkable capacity for developmental plasticity within each child and, in particular, how early environments can change the way that genes express themselves through epigenetic mechanisms. Epigenesis involves alterations to DNA other than changes in sequence that, nonetheless, can be passed on with cell division over the life course. For instance, when a certain DNA building block (cytosine) is methylated in a promoter region of the genome, this alteration may change the expression of the gene. We now know that methylation at many promoter regions is under environmental influence during pregnancy and early life (12
). Most important, it is becoming clear that differences in experience for young children across the social and economic spectrum of wealthy countries is capable of producing different epigenetic expressions. Epigenesis is one mechanism that can help explain biological embedding, but others are sure to follow.
Evidence showing that the early childhood years are more important than we previously understood comes from a wide variety of credible scientific sources, from cell biology to birth cohort studies. But the application of this knowledge to policy and programs has not kept up. In other words, there is a growing gap between what we know and what we do in the early childhood years, which needs to be closed. There is now a credible body of evidence showing that one new dollar spent in the early childhood years on quality learning, development, parenting and care programs has the largest economic return to society ($4 to $8 returned per $1 spent) of any new investment, even after discounting for inflation over the years and decades it takes for children to grow up (13
). This return is much greater than one new dollar spent between kindergarten and grade 12 or, for that matter, at the postsecondary level.