instinctive

spontaneous, impulsive, instinctive, automatic, mechanical mean acting or activated without deliberation.

spontaneous implies lack of prompting and connotes naturalness.

impulsive implies acting under stress of emotion or spirit of the moment.

instinctive stresses action involving neither judgment nor will.

automatic implies action engaging neither the mind nor the emotions and connotes a predictable response.

mechanical stresses the lifeless, often perfunctory character of the response.