Teens who have lived in poverty experience physical signs of stress at higher levels than those in more economically secure families. In an ongoing longitudinal study, researchers tested the saliva of 229 17-year-olds, many of whom had lived at or below the poverty level between the ages 10 and 16, for their levels of cortisol — a hormone in the blood. It is one of the commonest metrics for measuring stress. It showed that poverty in children has resulted in stress. Also, researchers examined cortisol levels after participants experienced a social stressor and their cortisol levels typically increase following stress.
The author develops the passage mainly by ___ .