Brain Development in Toddlers: What Happens and Why

The brain—unlike other vital organs in the body, whose organization and processes are completely functional at birth and then grow in a linear manner along with the body—is unfinished and immature in its development at birth. Brain development is front-loaded, with the majority of neurological systems being developed within the first three years of life.

This explosion of the brain and nervous system is constructed through interactions of our genetic programming and environmental influences. Over 70% of the brain’s organization takes place after birth. By exposure to patterned, repetitive stimulation from the external sensory and the internal biochemical environment, neurons are modified and the connections between them are established, strengthened, or eliminated. Neurons that are activated simultaneously form connections that increase the probability that these same neural patterns will be activated together in the future. Those cells or neural networks that are not activated die and get reabsorbed. This neurological architectural sculpturing process of overgrowth of neuronal synaptic connections, followed by a pruning back of unused cells is a principal labeled “use- or experience-dependent development.” Cells that fire together are wired to get together.

A two-year-old has twice as many synaptic connections as an adult. The incredible capacity of the brain to change in response to sensory information, store that input, and then use the information to guide our behavior allows humans to adapt to a wide range of environments. This ability to adapt is a gift but it also leaves children extremely vulnerable to long term consequences from maltreatment. The most powerful stimuli during this critical developmental period are the interactions with caretakers. The sensory interactions between a caretaker and an infant actually wire the brain and create the internal relational representations that will guide our behavior throughout life.

The brain, which is going through such explosive development from conception to age four, is most sensitive to exposure to growth facilitative or growth inhibiting experiences. Experiences we have early in life have the greatest impact on our long term cognitive, emotional and social functioning.  The experiences we expose our children to with the greatest frequency will influence not only their behavior but alter the way their genes are expressed and the way their brains process information, Two children with equal genetic potential, one raised in a safe, predictable and nurturing environment and one raised in a threatening, chaotic environment will have very different developmental trajectories. The first will be capable of tolerating stress, engaging in and succeeding in the complex social environments of school, peers and later work. The second child will struggle to feel safe, have trouble engaging others and maintaining healthy relationships and be at greater risk for mental health, academic, medical and substance abuse problems throughout their lives.

The best way to help children is through preventive measures that raise the awareness of the potentials and vulnerabilities of our children in our communities. By educating parents, grandparents, physicians, teachers, coaches, and other adults that interact with children to be sensitive to their developmental needs, we can increase exposure to those experiences that allow the expression of our children's genetic potential and minimize exposure to those that increase risk factors. For those unfortunate children exposed to abuse, neglect and violence, early identification and interventions from mental health professionals that are trauma-informed will have the greatest impact and result in best outcomes.