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. For example, the fight-or-flight response to danger is instinctive, as is the tendency for babies to cry when hungry. Instinctual, meanwhile, describes feelings, thoughts, behaviors, and tendencies born of deep, motivational forces. In other words, things that are instinctual tend to be much more complex than things that are merely instinctive. Still, while this distinction is important in some areas of psychology, instinctive is often used where instinctual would also apply.
Outside science, there are many claims about the supposed difference between the words: that instinctive is for isolated behaviors and instinctual is for patterns of behavior; that instinctive is more literal and instinctual is more figurative (i.e., synonymous with intuitive or deeply engrained); that the opposite is the case; that instinctive does not necessarily relate to instinct and is often synonymous with reflexive; that instinctive is more British and instinctual is more American; that instinctual is preferred in scientific usage, while instinctive is the popular term; and that instinctual pertains to human psychological instincts and instinctive to more animal instincts. Any one of these distinctions might be useful if it were borne out broadly in the language, but none is.
What we can say, however, is that instinctive is far more common and many centuries older. Instinctual came about early in the 20th century and initially appeared mostly in texts on psychology—the word appears especially often in translations of Freud and in references to his work—so it makes sense that the fine distinction between the words is now observed in that field.
When the temperature drops, it is instinctual to seek warmth as soon as possible. [Purdue University Calumet Chronicle]
It is instinctive for us to seek a grand, moralistic mind that is not there. [Guardian]
The connection between touch and understanding is deeply instinctual, beginning in infancy and continuing, in varying forms, throughout our lives. [Scientific American Mind]
Economic exchange has been going on throughout human history, so it is possible that our ancestors evolved an instinctive capacity for recognising the difference between situations suited to social or market norms. [New Scientist 1]
Sigmund Freud’s psychoanalytic theory views motherhood as based on innate instinctual drives that are normal characteristics of a woman’s female identity. [Encyclopedia of Motherhood]
According to this view, there is no instinctive human programming for fistfighting, pipe bombing, knife wielding, gun loading, 95-mile-an-hour “bean balls,” or other violent or aggressive behaviors. [Psychology: A Journey Dennis Coon and John O. Mitterer]
1. Mark, Buchanan. n.d. “Cover Story: Money in mind.” New Scientist 201, 26-30. ↩