Definition
Gene-environment interaction is the principle that a trait emerges from genes and environment acting together, such that the effect of a gene depends on the environment and the effect of an environment depends on the genes present.
Sapolsky's reframing is blunt: asking whether a behavior is genetic or environmental is the wrong question. It is roughly as sensible as asking whether the area of a rectangle is caused by its length or its width. The honest answer to "what does this gene do?" is always "it depends on the environment."
Why it matters
How it works
Most gene-environment interaction operates through gene regulation. An environmental signal — abuse, nutrition, social rank, a stress hormone — alters which transcription factors are active or leaves an epigenetic mark, and that changes how strongly a gene is expressed. A genotype that looks identical on paper therefore yields different outcomes in different surroundings, because the surroundings governed how that genotype was read.