Devices, systems and methods are disclosed for treating or preventing an autism spectrum disorder, a pervasive developmental disorder, or a disorder of psychological development. The methods comprise transmitting impulses of energy non-invasively to selected nerve fibers, particularly those in a vagus nerve. The nerve stimulation may be used as a behavior conditioning tool, by producing euphoria in an autistic individual. Vagus nerve stimulation is also used to modulate circulating serotonin levels in a pregnant woman so as to reduce the risk of having an autistic child； modulate the levels of growth factors within a child； promote balance of neuronal excitation/inhibition； modulate the activity of abnormal resting state neuronal networks； increase respiratory sinus arrhythmia； and avert episodes of motor stereotypies with the aid of forecasting methods.
