When the parent also transmits what explains the effect

Parents transmit both an environment and genetic material. An association between what they do and what the child becomes may therefore reflect a transmission that does not run through the observed practice. A parent who reads a great deal may both fill the house with books and pass on dispositions that favour reading, without the books themselves playing the role attributed to them.

The reasoning applies to many studied associations: number of books at home, participation in activities, conversational style. None of those variables is independent of the parents' characteristics.

The literature distinguishes a passive correlation, where parents provide an environment correlated with what they otherwise transmit; an evocative correlation, where the child elicits responses; and an active correlation, where the child selects their own environments. That typology was formulated in 1977 by Robert Plomin, John DeFries and John Loehlin.

All three produce measurable associations without any environmental effect being involved, which makes causal interpretation of ordinary family studies very fragile.

A child with a very active temperament, for example, elicits different responses from a calm child; a measure of parental behaviour then partly records the effect of the child on the parent, and not the reverse.

Adoption studies compare children raised by adults with whom they share no genetic material. They find environmental effects, generally weaker than the associations measured in ordinary families. They have their own limitations, notably the selective placement of children and the fact that adoptive families differ from the general population in their stability and resources.

Comparisons between siblings within the same family control for everything the family shares. They sharply reduce many published associations, without cancelling them all.

More recent work uses parents' genetic data to estimate the effect of variants they carry but did not transmit. Such an effect can only run through the environment they create. The approach was popularised by an article published in 2018 by Augustine Kong and colleagues, under the name genetic nurture.

Those analyses find non-zero contributions, which documents a real environmental route, while showing it is smaller than raw associations suggested.

That raw associations overstate the effect of the environment does not establish that the environment does not matter. Designs that control for transmission find reduced but present effects.

The practical consequence is a reading requirement: a study measuring a parenting practice and a child outcome in ordinary families cannot, on its own, establish an effect of the practice.