Researchers have long recognized that not everyone abused or neglected as a child goes on to have problems. In some cases, genes can buffer against environmental effects and allow the brain to develop normally; in others, those that might otherwise be silent are triggered by adversity in early childhood and so influence brain development.
As an analog for more complex human behaviour, Prof. Sokolowski studies fruit flies that naturally carry one of two possible variants of a gene that affects food-foraging habits. The genes, dubbed “rover” and “sitter,” will either lead a fly to wander around a food source or stay put and feed. Fruit flies that are genetically predisposed to be rovers nevertheless become sitters if they are nutritionally deprived during development. The effect of the scarcity is to ramp down the activity of the rover gene, thereby maximizing food intake over other kinds of behaviour.
At McGill University, a powerful set of studies by behavioural scientist Michael Meaney and his colleagues has been especially important at showing how the interplay of genes and social environment can program the behaviour of mammals.
Mother rats who lick and groom their babies less often tend to produce offspring more sensitive to stress. The effect is thought to be “epigenetic” – the underlying DNA sequence of the baby rats is unchanged, but a cascade of biochemical signals triggered by the grooming affects the activity of a gene that is crucial for regulating stress response.
The experiments suggest how the social adversity that can come with low socio-economic status may work on human children, Prof. Sokolowski says, by reducing the signals of key genes that guide brain development “like a dimmer switch.” This, in turn, affects cognition and behaviour with consequences that can reverberate through a lifetime.
Such vulnerability may showcase a fundamental weakness in the way our brains work. It may even be evolution’s way of preparing brains for the environments they are growing up in.
“The brain is not fragile; the brain is adaptive,” Dr. Herztman says. “The question is whether or not those adaptations will allow you to cope with the world you are then going to live in.”
Thus, a developing brain that has been influenced by a stressful or chaotic social environment at an early age may lead to a child with serious attention issues in the classroom years later.
But a behaviour pattern that is perceived as a deficit in the school setting may be there precisely because the child, as an infant, was shaped by social circumstances to pay more attention to distractions that could warn of sudden danger.
“It’s a kind of vigilance – the brain is searching for threatening stimuli to thwart,” Dr. Hertzman says. But the epigenetic fine-tuning comes at a cost, he adds, because the brain is less able to concentrate on high-order functions, such as math or reading.
“The problem is that the developmental signals and the demands of modern society become a mismatch.”
Symptoms of such cognitive disorders as attention deficit and hyperactivity disorder (ADHD) are not restricted to children of low socio-economic status, of course, and do not always relate to parenting. However, evidence that the environment plays a role in the prevalence of such symptoms is reinforced by studies that track individuals over the course of their lives.
One such study, based in Wisconsin, has followed more than 500 children from the second trimester of pregnancy through high-school graduation.
Those who were in preschool at a time when their parents reported high levels of economic and social stress bear the scars of that stress in the form of epigenetic marks on their DNA. These marks will persist for life, inhibiting genes that might otherwise be more active.
Thomas Boyce, a professor of pediatrics at UBC, has collaborated with the team behind the Wisconsin study, and says its findings could guide strategies to head off the negative impact of cognitive differences even before it becomes apparent.
Prof. Boyce (who leads with Prof. Sokolowski a long-running program in experience-based brain and biological development sponsored by CIFAR) and his colleagues at UBC are currently studying children from different socio-economic backgrounds in the Vancouver area.