Definition: Individual differences are the variations from one person to another on variables such as self-esteem, rate of cognitive development or degree of agreeableness. Historically, psychological science has overlooked individual differences in favor of focusing on average behavior.
For instance, we know that, on average, girls first experience signs of puberty around 10.5 years of age. While this is important information, it's also valuable to consider differences in pubertal development. Some girls experience puberty very early or very late. Psychologists have realized that either situations can have major consequences for the girl's future. If we only studied the average-in other words, if we overlooked individual differences-we would miss out on key information about child development.
Individual differences have been most often studied in the area of personality development. Psychologists have collected vast amounts of data on how people vary from one another in terms of their traits. For instance, they have noted that individual differences on the "Big Five" personality traits first strongly appear during the tween years.
All in all, the study of individual differences helps us to understand not only what makes humans similar to one another, but also what makes them different. By considering the variations that can occur from one person to another, we can best understand the full range of human behavior. We can also come to understand what constitutes normal variation-such as starting puberty at 9 years of age instead of 10.5 years-and which developmental rates may be red flags for intervention, such as in the case of learning disorders.
Berger, Kathleen. The Developing Person Through the Lifespan. 2008. 7th Edition. New York: Worth.