Constancy and change in the growth of human characteristics.

Neither genetic programmes nor social influences necessarily unfold in a constant way, and their interactions are complex. The role of chance events adds a further and sometimes potent uncertainty in prediction equations. Both constancies and changes of ordinal position and/or level occur for most characteristics in normal circumstances but, following significant ecological improvement, personal changes among the disadvantaged can be much larger. Recent research emphasises the inadequacy of considering either genetic or environmental effects during one period of development outside the context of preceding and subsequent influences. It is to long-term consistent influences that importance must be ascribed.