Experience-based brain and biological development in the early years differentiates neuron functions and establishes major neurological pathways. This can set trajectories that affect the competence, health and well-being of individuals throughout life. We now better understand how the social environment in early life gets under the skin to affect brain development. This knowledge has implications for medicine, particularly in terms of the role of public health, family medicine and developmental paediatrics.
The brain controls, through neurological and biological pathways, the main functions of all mammals (metabolism, reproduction, respiration, the cardiovascular system, the immune system, emotions, sex, behaviour, response to stress and threats, learning and other functions). The billions of neurons in the brain have the same genetic coding, but as the brain develops through experience in early life, neurons differentiate through specific gene activation. Experience also affects the formation of the connections (synapses) among neurons to establish pathways for the different hierarchies of brain function. These pathways govern or control our intellectual, emotional, psychological, physiological and physical responses to what we do every day.
Genes, in addition to being activated, can be deactivated more or less permanently by epigenetic processes that affect gene (DNA) function. Studies of the 1970 Dunedin birth cohort and studies in monkeys and rats illustrate what appears to be an epigenetic process affecting neuron function contributing to behaviour problems and depression. Young children who were raised in a poor environment with one or two copies of the short allele of the serotonin gene promoter polymorphism were at risk for depression in adult life. Those with the short gene structure brought up in a good environment were not at risk. Those homozygous for the long allele serotonin transporter gene structure were resistant to the adverse effects of poor early child development (resilient). Of importance from this work is the demonstration of a nongenotype mechanism for transmitting patterns of behaviour for the genetically vulnerable to the next generation.
The development of the limbic hypothalamus pituitary adrenal (LHPA) pathway in early life has long-term effects on behaviour and cognition. We use many descriptive terms for behaviours that relate to the function of the LHPA frontal brain pathway. Attention-deficit hyperactivity disorder (ADHD) is an example. This appears to be a product of the interaction between the environment and genetic vulnerability, and between the LHPA and frontal brain pathways. Environmental factors that contribute to ADHD in vulnerable individuals include pregnancy and delivery complications, prematurity and a dysfunctional family environment. The brain pathways involved in ADHD are also involved in other forms of behaviour (comorbidity). Thus, ADHD is associated with psychiatric disorders and substance abuse.
There is now considerable evidence that cardiovascular diseases (coronary artery disease, high blood pressure, type 2 diabetes, immune disorders, obesity, psychiatric disorders and other problems) are linked to early brain development in utero and early life. The brain’s biological pathways that are related to risk factors for coronary artery disease, such as cholesterol and smoking, are becoming better understood.
Language and literacy assessment is a good measure of overall brain development in the early years. The sounds that an infant is exposed to when very young influence how the auditory neurons differentiate and function. For example, infants exposed to two languages (for example, Japanese and English) in the first seven to eight months of life set the base for fluency in the two languages with no accent. Individuals who master two languages early in life have a larger left brain hemisphere. Children who develop poor verbal skills during the first three years of development will tend to do poorly in language and literacy in the school system, and children who develop poor verbal skills in the first two years are at risk for antisocial behaviour as teenagers. Why is there a relationship between literacy and language development in the early years and antisocial behaviour later in life?
The Organisation for Economic Co-operation and Development studies on literacy (1) show that between 42% and 48% of individuals aged 16 to 65 years in Canada and the United States performed poorly on the literacy assessments (categorized as Levels 1 and 2). At this level, individuals have trouble understanding prescriptions or consent forms. In today’s world, it is argued that populations should be at Level 3 to be able to function adequately.
In Canada and the United States, between 15% and 20% of the population scored at Levels 4 and 5 (high). In contrast, in countries like Sweden, 34% are at Levels 4 and 5. Because Sweden’s base for language and literacy is set in the early years and because it has good early child development programs, this may account for the high levels achieved.
Another subject of interest is the relationship between literacy and health status. In the United States, 50% of the least literate (Level 1) individuals have poor physical and mental health. The relationship between adult health status and literacy is a gradient. Fewer than 2% of individuals at Level 5 have poor health. The evidence is strong that experience-based brain development in the early years sets brain and biological pathways that affect health (physical and mental), learning and behaviour throughout life.
Early child and brain development is profoundly affected by the quality of caregiving (including parents) and support in the early years, including pregnancy. Initiatives to ensure high-quality early child development require the involvement of parents along with appropriate institutional support. In a report to the Government of Ontario in 1999 (2), McCain and Mustard set out a plan for early child and parenting centres to improve early child development. A recent report from the Brookings Institution in Washington, DC, by Ludwig and Sawhill emphasized the importance of quality early child development programs.
They emphasized the following points:
- Intervene early;
- Intervene often; and
- Intervene effectively.
Programs that enhance early brain development are important if we wish to establish prosperous, healthy, equitable, tolerant, pluralistic, sustainable, democratic societies. Unless we find strategies to improve the health, quality and competence of populations by investing in early child development, societies will run the risk of slipping into chaos, with negative effects on our continuing experiments in civilization.