From the work of Piaget and others evolved the notion that, if child development follows a particular trajectory or sequence of stages, then one can measure the attainment of milestones and find children who are ahead of or behind a "typical" pace in their development. This theory of cognitive development has been supported and extended by research on intelligence and the evidence that individuals become more intelligent – that is better at solving problems or thinking in certain ways – as they get older – at least up to a certain age.  Thus, assessments of intelligence involve comparison to a peer group of some kind.

Research and debate on the nature of intelligence has been extensive, particularly on the relative contributions of nature (genetics) and nurture (the environment) and on whether intelligence represents a single capacity or multiple capacities.

The nature vs. nurture argument, as it relates to cognitive development, has been largely put to rest.  The broad consensus is that the interaction of genetic and environmental contributions both influence an individual’s level of intelligence, and that nature and nurture interact in complex ways.

Efforts to define intelligence have yielded significant diversity of opinion.  Characterizing intelligence as one thing no longer has the support it once did.  The idea that intelligence comprises multiple capacities is broadly accepted.  Since the evidence supports the role of many different cognitive skills in learning, we generally refer to cognitive skills rather than using the term "intelligence."  Thus cognitive development can be thought of as the development and maturation, through interaction with the environment, of the cognitive skills that make us effective in school, work and our daily lives.