Twin Observations: What Psychology Can Learn from Twins

They're mirror images, but twins can be astoundingly different. How twins have helped psychology address the nature vs. nurture debate.

By Dana Frisch, published on May 01, 2003 - last reviewed on July 28, 2009

Despite obvious similarities, their differences can be astounding: One twin is right-handed, and the other is a lefty. One is outgoing, and the other is reserved. Some of the differences are apparent early in age, whereas others develop over time as genes and the environment interact.

Arnold Gesell, Ph.D., M.D., was a pioneering researcher who studied twins to understand this phenomenon. Gesell, a child psychologist who established the Yale Clinic of Child Development, showed that behavior develops in a genetically predetermined way. In one 1929 study, he compared a pair of twin infants in the task of stair climbing. One twin was trained, while the other wasn't. In a short time, the untrained twin learned the skill on his own, and weeks later both twins exhibited the same proficiency. There is, he showed, an inherent progression in the development of physical skills, which emerges even without training.

Gesell also contributed to psychology through his methods of "cinemanalysis." In his studies, children were filmed through a one-way screen, enabling him to later analyze emerging behavior patterns in slow motion. The endless footage helped establish the Yale Films of Child Development. His work gained mass appeal and helped pave the way for modern-day child psychology. In 1940, Time magazine named Gesell the world's best-known expert on child behavior.

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