The article was updated on June 18, 2015.

Harvard professor Jack Shonkoff is talking about the profound effect parents have on their young children when he mentions the "Still Face Experiment", videos of which have become something of a YouTube sensation.

Done by developmental psychologist Edward Tronick who now teaches at the University of Massachusetts in Boston, the tests illustrate how a parent's interactions with his or her child can have a powerful effect.

In one of the YouTube videos viewed by millions, a mother is seen cooing, talking and laughing with her one-year-old child. But when the mother suddenly stops responding, the baby immediately registers something is amiss and with increasing desperation tries to win back the mother's attention. When all else fails, the baby collapses in tears.

Speaking at Harvard University's Center on the Developing Child, which he heads, Professor Shonkoff refers to the experiments to explain "toxic stress" - a term coined by the National Scientific Council on the Developing Child, which he chairs. It describes the chronic stress suffered by children who face various adversities in childhood.

"Just like in the earlier part of the video, when the interactions between a mother and child are positive, it helps fuel an explosion of neural circuits in the infant, linking disparate parts of the brain and growing stronger as learning and experience reinforce connections.

"The child's interactions with her parents - being fed, hugged, cooed at, sung to - send electrical impulses shooting through the developing circuits of her brain, strengthening pathways and inciting new synapses to grow. At its peak, the cerebral cortex region of an infant's brain can produce 700-1,000 new synapses every second. All this develops the child's capacity to think, learn and process emotions," he says.

A whole host of problems arise when interactions between parent and child are poor or, worse, non-existent.

Referring to the second part of the video, when the child is stressed by the mother's sudden unresponsiveness, he says: "When a young child's stress response systems are activated within an environment of supportive relationships with adults, these physiological effects are buffered and brought back down to baseline. The result is the development of healthy stress response systems.

"But in situations of chronic neglect or abuse, where there is no adult presence to comfort and calm the child, her stress response can stay elevated. And, over some time, this turns into toxic stress, which research has shown can impair a child's capacity to learn, and even her physical and mental health."

The effects can last a lifetime, he warns.

"Just as a weak foundation compromises the quality and strength of a house, adverse experiences early in life can impair brain architecture, with negative effects lasting into adulthood," he says, citing another landmark study.

The Adverse Childhood Experiences (ACE) Study is one of the largest investigations ever done in the US to assess associations between childhood maltreatment and later-life health and well-being. It involved more than 17,000 people who provided detailed information about their childhood experiences of abuse and neglect as well as family dysfunction.

The findings suggest that certain experiences are major risk factors for leading causes of illness such as diabetes, heart disease and stroke.

Prof Shonkoff says toxic stress affects the learning capacity of children, which subsequently leads to underperformance in school, and stunts growth in parts of the brain.

It affects the prefrontal cortex, the area of the brain most closely associated with "executive function" skills, which are not the same as intelligence but encompass abilities crucial to learning, such as paying attention and following directions.

That means that when children start school, a gap has already opened up between pupils from unstable home environments and their better-off peers.

Prof Shonkoff refers to many early childhood programmes that focus on providing language exposure and says that while early literacy experiences are important for young children, they are no more important than paying attention to their social and emotional well-being.

Neuroscience has shown that emotional experiences, the quality of the relationships that children have with the important people in their lives and the interactions and feelings that go with those relationships actually influence learning.

He cites reading as an example.

"There is a significant amount of intellectual development and language development that you need to master reading. But a child's ability to read also depends on whether he can sit still, if he is preoccupied with feelings of sadness or anxiety and if he can control his impulses," he says.

Then it is not only the child's reading that suffers, but also his ability to learn as a whole.

"If a child can't sit still, cannot wait his turn, or can't function well in a group or control his impulses, or deal with other children, he is not going to succeed in school."

Fortunately, there is a silver lining. Research has found there are ways to lessen toxic stress.

The best and most effective way is to reduce the chances of young children being exposed to extremely stressful conditions, such as recurrent abuse, chronic neglect and violence at home.

Prof Shonkoff urges governments to start programmes such as home visits to teach parents about providing the right kind of environment. There should also be cheap, if not free, childcare centres for children from disadvantaged homes.

"Research shows that even under stressful conditions, supportive, responsive relationships with caring adults as early in life as possible can prevent or reverse the damaging effects of toxic stress response," he says.

It is also critical to intervene early, in the crucial window when the brain is developing.

"There's a lot of research that tells us we are missing a critical, time-sensitive opportunity to help the most disadvantaged kids. The evidence suggests that we must start early, even before a child is born. That means telling about-to-be mothers about the dangers of drug use, alcohol and tobacco," he says.

Children's programmes are most successful when parents are on board. "You can't leave the parents out of it," he says. "There should be programmes to give parents the tools to nurture their child."

"Having a nurturing mother almost completely mitigated the developmental damage that, in other children, correlated with stress," Prof Shonkoff says.Studies have found that mothers rated as being particularly responsive and nurturing had babies whose cortisol patterns were much more likely to be normal, regardless of whether they lived in poverty or chaos. Even in cases where these brain patterns were irregular, those with responsive mothers were likely to score higher on tests of executive function.

Urging governments to pay attention to children faced with neglect, even abuse and violence, he says: "The consequences of toxic stress are among the most expensive problems that a society deals with. Prisons are far more expensive to run than early childhood programmes. Economic dependence is much more costly than people who are productive, who earn a living and pay taxes. Being healthy is much less expensive than paying for the treatment of heart disease and diabetes."

He says there is an urgent need to respond to "the scourge of childhood neglect".

Governments and societies cannot afford to ignore the science on the damage that toxic stress causes in children.

"You look at the mounting evidence. We can't allow this to go on.

"The future of any society depends on its ability to foster the healthy development of the next generation," he says.

sandra@sph.com.sg

Note: The Center on the Developing Child has clarified that Prof Shonkoff urges policymakers and community leaders to provide programmes that build caregivers' skills for parenting, maintaining a stable household, and getting and keeping a job. While these environments can be provided in the home and in child care programmes, the emphasis needs to be on the quality of the environments and the skills of the teachers.