Table 2.

Final Model for Conventional Volumes^a

Variable	df	F Score	P Value
TS	1,275	7.67	.006
OCD	1,275	0.11	.74
ADHD	1,275	0.01	.91
Age	1,275	0.06	.81
Sex	1,275	1.63	.20
Region	1,280	884.88	<.001
Hemisphere	1,281	4.68	.03
WBV	1,275	89.59	<.001
Sex × region	1,280	9.80	.001

Abbreviations: ADHD, attention-deficit/hyperactivity disorder; df, degrees of freedom; OCD, obsessive-compulsive disorder; TS, Tourette syndrome; WBV, whole-brain volume.

^aThe model was determined through a procedure in which all main effects were forced into the model and higher-order terms were removed via backward stepwise selection with the constraint that the model was hierarchically well-formulated at each step. The significant main effect of TS indicated differences in volume across diagnostic groups (TS vs controls). Additional significant terms included region, indicating that the amygdala and hippocampus, on average, differed significantly from one another in their volumes; WBV, indicating the presence of scaling effects in the data (the larger the brain, the larger the amygdala and hippocampus); and the sex × region interaction, indicating that a difference in volume across sexes varied by brain subregion (amygdala or hippocampus). This regional specificity of sex differences did not vary by diagnosis, however (ie, the sex × region × TS interaction was not significant).