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The direct observation of identical twin girls from birth through the fourth year is reported in detail. One girl, smaller from birth and slower in developing motor coördination, showed greater and more enduring social responsiveness
than the other who showed more perseverance and self-sufficiency. The personality differences appeared to arise from physical differences and these in turn from differences in intrauterine environment rather than from later influences. If this is so, then such theoretical concepts as Freud's 'congenital ego variations' and Hartmann's innate maturational tendencies require revision so as to include the influence of antenatal experience.