Timing and Critical Periods

Key Takeaways

• Our biological sensitivity to environmental influences varies across the life course, making the timing of when we encounter those influences particularly important.  
• The sensitivity of developing biological systems is typically greatest in the prenatal period and decreases over the course of childhood and into adulthood.  
• Just as negative experiences and exposures are more likely to have harmful impacts in early childhood, anything we do to protect children during that time is likely to have significant positive effects on development.

Fast Facts

Humans differ in their sensitivity to environmental influences at various points in their lives. The sensitivity of developing biological systems is typically greatest in the prenatal period. Young children are more biologically sensitive to most adverse exposures than adolescents, who are, in turn, more sensitive to most exposures than many adults. 

During pregnancy, the immature biological systems of a growing embryo develop at an extremely fast pace, and their development is powerfully shaped by interactions with the environment around them. These systems “read” conditions in the womb as predictors of what they will encounter after birth and adapt accordingly. This makes these developing biological systems more sensitive to positive and negative environmental influences compared to later in life when they have matured. 

For example, the effects of certain types of air pollution on development can vary greatly depending on the timing of exposure. Prenatal exposure can result in babies being born too early or at low birth weight. Whereas later in childhood, exposure to air pollution can lead to an increased risk of developing childhood leukemia. These differences in outcomes demonstrate how the effects of a particular environmental influence will vary depending on the timing of the exposure during development.  

All of a child’s experiences and exposures, along with their timing, interact with their genes to shape the development of their biological systems, including the brain. In other words, what children encounter in their environment and when they encounter it interact with how their genes are expressed in powerfully connected ways. For example, gene expression can change in response to positive environmental influences—such as supportive relationships and a clean water supply—as well as their absence. This underscores the importance of providing safe, supportive developmental environments for children in the earliest years when the brain and other biological systems are developing most rapidly. It is also important to recognize that the conditions of children’s environments are not distributed equally. They have been designed by decisions made over time and can be re-designed to address disparities and provide healthy developmental environments for all our children.