A number of theologians and philosophers make the claim, implicitly if not explicitly, that having free will is essential to human nature. This is, perhaps, a fairly natural claim, particularly if one thinks that free will is a capacity of the human soul. But the claim got me thinking, as there may be a counterexample.
The first potential counterexample will depend on the details of what one's view of free will is. Consider, for example, John Fischer's view according to which free will requires a certain level of ability to recognize and respond to moral reasons. But then what about psycopaths, who are incapable of recognizing and/or being moved by certain sorts of moral reasons, namely those that pertain to the good of other individuals? Even if psycopathy renders individuals who suffer from it not morally responsible, it would seem odd if they weren't human. Now, perhaps psycopathy at most takes away certain aspects of one's ability to recognize or be moved by certain moral reasons, but it leaves one's abilitty to recognize and be moved by other kinds of moral reasons intact. So perhaps what psycopathy does is limit the range of one's free will, but doesn't diminish it all together.
The second potential type of counterexample is young children. My daughter is currently only ten months old, and I strongly doubt she has either the volitional or intellectual capabilities for free will, although she likely will once she reaches a certain age. In response to this, perhaps one could say that what is essential to humans is not the actual having of free will, but the capacity to have free will.
But finally, consider those humans who have genetic disorders that strongly impair their intellectual and/or volitional capacities. Such individuals may not reach the level of these capacities needed for free will even once fully grown. Nor, given their genetic disorders, does it look like even have the capacity for free will in the way suggested regarding young children.
So it looks like the claim the free will is essential to human nature is false.