Instinctive is often used to mean “controlled by instincts” when describing a specific behavior (as in “an instinctive reaction”). Instinctual has the more analytical meaning “relating to instinct,” often used by scientists when identifying a process or pattern rather than any specific behavior.

At root, instinctive and instinctual are essentially the same; both mean (1) of or arising from the instinct, or (2) pertaining to the instinct. There is a subtle difference between them in some writing on psychology published in the last century. In these contexts, instinctive describes any unlearned response no matter how basic.

Both are derived from the noun “instinct,” and Buck’s many dictionaries give them similar meanings. The Oxford English Dictionary defines “instinctual” as “relating to or prompted by instinct; apparently unconscious or automatic.” It defines “instinctive” as an adjective form of “instinct.”

The choice between "instinctually" and "instinctively" can also be influenced by the context and the speaker's preference for the sound or rhythm of the language, with "instinctively" being more commonly used due to its familiarity and ease of pronunciation.