Abstract: The personality structure of 36 pairs of identical twins reared together was determined using The Essential Enneagram Test, (Daniels & Price, 2000) plus interviews of the participants and their family members. The results were compared with results from pairs of randomly selected unrelated individuals of known types. The Chi Square Test showed significant differences between the identical twins and the random pairs (p=0.003). Contrary to expectation, only 2 of the 36 pairs had the same personality type as each other (5.5%), fewer than in the control group (16.8%)(p=0.089). However, the personalities of 30 pairs were connected in several other ways on the Enneagram diagram. We hypothesize that these findings reflect both a strong genetic influence and an environmentally based “drive” for differentiation between identical twins reared together.