This randomized control trial (RCT) will test the hypothesis that physiological response patterns of salivary cortisol, cardiovascular, and electrodermal activity account for our previously observed significant outcomes (i.e., reduced irritability and hyperactivity, and improved social and communication), and additional outcomes (emotion regulation caregiver quality of life and crisis mental health care usage), in youth ages 6-16 yrs. with ASD and co-occurring psychiatric diagnoses randomized to a 10-week manualized THR intervention compared to a no-horse Barn Activity (BA) control (Aim 1). We will evaluate the durability of Aim 1 outcomes in the THR group compared to the BA control group six-months after the intervention period (Aim 2). Finally, we will explore dose and sub-population effects of THR and BA interventions by comparing effect size differences in THR and BA groups to (a) a 10-week wait-list control group; (b) a Hybrid intervention group (five weeks BA followed by five weeks THR); and (c) a subsample of the THR study population randomized following psychiatric hospitalization (Aim 3).
