Whether the children had too little or too much cortisol, they demonstrated below-average cognitive ability at age 4. The higher or lower levels were linked to specific forms of family adversity: "children with family instability or harsh and emotionally distant caregivers at age 2 had elevated cortisol levels, while children with only family instability at age 2 had lower than average cortisol levels," the study said. The researchers, who defined family instability as "frequent changes in care providers, household members or residence" that can "reflect a general breakdown of a family's ability to provide a predictable and stable environment for the child," were surprised to find that the cortisol levels remained relatively stable throughout the study.
As coauthor Melissa L. Sturge-Apple, assistant professor of psychology at the University of Rochester, explained, "The exact mechanisms through which too much or too little cortisol affects cognitive functioning aren't fully understood. Researchers hypothesize that too much cortisol can have toxic effects on parts of the brain that are important for cognitive functioning, and too little might hinder the body's ability to recruit the biological resources necessary for optimal cognitive functioning."
Lead author Jennifer Suor, a Ph.D. candidate in clinical psychology at University of Rochester, added, "What we were interested in seeing is whether specific risk factors of children living in poverty might be related to children's cortisol levels. Then we looked to see if the hormone levels are predictive of significant differences in the children's ability to think."
Describing cortisol as "public health enemy number one, Christopher Bergland, writing in a blog from Psychology Today, related that scientists have known that elevated levels of the hormone "interfere with learning and memory, lower immune function and bone density, increase weight gain, blood pressure, cholesterol, heart disease..." and "increase risk for depression, mental illness and lower life expectancy." Bergland explained that the adrenal glands release cortisol as a response to fear or stress "as part of the fight-or-flight mechanism," which was an adaptation defined in detail by Canadian biochemists Hans Selye of McGill University in Montreal, Canada (https://www.psychologytoday.com/blog/the-athletes-way/201301/cortisol-why-the-stress-hormone-is-public-enemy-no-1).