Two infants in a rural setting achieve walking at different ages despite…

Concept: Development follows a set of general principles. One of them, the principle of individual differences, holds that although the sequence of developmental milestones is broadly the same for all children, the rate at which a child moves through that sequence varies, because heredity, maturation, nutrition and opportunity for practice differ from child to child. A milestone age is therefore stated as a normal range, not as a fixed point.

Application: Here both infants live in the same rural setting and follow similar family routines, so the broad environment is roughly held constant. One walks at ten months and the other at fourteen months. Both ages fall inside the accepted normal range for independent walking (about nine to sixteen months), so what the observation records is not a different course of development but a different pace of reaching the same milestone. Variation in the rate at which two children reach the same milestone is precisely what the principle of individual differences names, so the observation best demonstrates individual differences.

Cross-check: the other principles named in the options describe different axes of development, none of which is about the pace at which two different children reach the same milestone.

• Orthogenetic principle — development moves from a global, undifferentiated condition towards greater differentiation and hierarchic integration; it is about how behaviour becomes organised within one child.

• Cephalocaudal progression — motor control spreads from head to foot; it is about the order in which body regions come under control within one child, which is why head control precedes walking in every child.

• Integration — separately acquired specific movements are combined into coordinated wholes; it is about how skills join up within one child.

Result: two infants reaching the same milestone at ten and at fourteen months illustrate individual differences in the rate of development.