In its weakest form, it is assumed that a precursor or antecedent is simply a forerunner to some later-occurring event in development, without the implication that there is a functional connection between them. In the stronger form, a precursor is a necessary condition for that later event, which would not occur without it. In human development, it notoriously difficult to establish functional connections between earlier and later behaviours in the stronger sense, even when they are seemingly the members of the same developmental sequence. In most cases such connections have been assumed on the basis of similarities in form and timing between earlier and later behaviors. To do so, ignores a class of events termed ontogenetic adaptations: transient age-specific structures and functions that are only adaptive for a restricted phase of development and which may be unnecessary or even incompatible with adaptation at later phases. The Bowlby-Ainsworth theory of attachment has been criticised in a similar way. While it treated adult attachments as a continuation of child attachments, others have argued that latter must disappear before adulthood attachments are possible.
See Attachment theory, Necessary and sufficient conditions, Ontogenetic adaptation, Structure-function relationships