Table I.

Interpretation of possible genetic factor structure patterns

Factor structure	Consistent explanatory models
1. Ventricles load positively on same factor(s) as brain structures	Genes determine size of cranium and hence all subcortical structures and spaces
2. Ventricles load negatively on same factor(s) as brain structures	Genes determine size of gray matter structures; ventricular volumes determined through ex vacuo effects of these genes
3. Ventricles load on different factor(s) from brain structures	Different genetic influences determine gray matter versus ventricular volume
4. Single brain factor*	One set of genes determines the size of all subcortical structures, perhaps genes involved in global growth promotion
5. Multiple brain factors*
5a. All structures load positively on the factors	Different genetic influences act on different subcortical structures due to distinct:
	– phylogenic patterns
	– early developmental trajectories
	– functional connectivity (including anatomical connectivity and/or neurotransmitter systems)
	– ongoing cell repair or cell death mechanisms (e.g., nerve growth factor)
	– sensitivity to effects of stress, age, or pathology
5b. Some structures load negatively on factors	As above, but with additional evidence that genes act in a zero‐sum manner for neighboring structures (e.g., greater growth of the hippocampus means less growth of the amygdala)

^*Each of these possibilities could occur with any of the ventricular patterns above.