What do we call reactions that seem automatic and do not require much thought?

The term that accurately describes reactions that seem automatic and do not require much thought is "reflex actions." Reflex actions are involuntary responses to specific stimuli. They occur rapidly and automatically, often as a protective measure, without the involvement of consciousness or deliberate thought. For instance, when a doctor tests a patient's reflexes by tapping the kneecap, the leg jerks forward almost instantaneously, demonstrating this automatic response.

While simple reactions might also suggest a straightforward or quick response, this term does not fully capture the involuntary nature inherent in reflex actions. Complex reactions typically involve a greater degree of thought and multiple steps, which contrasts with the immediate nature of reflex actions. Instinctive responses, while also automatic, often imply a more complex biological or emotional background rather than the straightforward, immediate nature of a reflex. Thus, reflex actions are the most precise term to denote those reactions that happen automatically and without much conscious effort.