[Of moral philosophy] Nietzsche's general view is that moral judgements, not only those of the philosophers, are "just sign language of the affects", a view recognizable to any student of contemporary psychology as winning support from, for example, Jonathan Haidt's work on moral judgement, such as his well-known paper "Ther Emotional Dog and Its Rational Tail" (2001).
[Of the unconscious] The Nietzsche who laments, in The Gay Science, the "ridiculous overestimation and misunderstanding of consciousness"--when, in fact, "the greatest part of our spirit's activity remains unconscious and unfelt"--might be thought, by a contemporary reader, to have offered here an apt precis of Timothy Wilson's influential book Strangers to Ourselves: Discovering the adaptive unconscious (2002).
[Of free will] If the experience of willing does not, according to Nietzsche, illuminate how actions are brought about, what, then, really explains our actions? The "Four Great Errors" section of Twilight of the Idols suggests an answer, one which wins powerful support from recent work by the psychologist Daniel Wegner in his widely discussed book The Illusion of Conscious Will (2002).
[Of the inheritability of character traits] But, says Nietzsche,
a well-turned out human being . . . must perform certain actions and shrinks instinctively from other actions; he carries the order, which he represents physiologically, into his relations with other human beings and things.
. . . As it happens, something like Nietzsche's view seems, once again, to win support from recent empirical research. Two kinds of evidence are pertinent here. On the one hand, there is the famous work by Jerome Kagan on the biological foundations of temperament, suggested by the clear patterns of emotional and behavioral response apparent in very young children, patterns that often persist into maturity. A second kind of evidence, as Joshua Knobe and I have argued elsewhere, comes from studies in behavioural genetics involving twins . . . and adopted children, which show that every personality trait studied to date is heritable to a surprising degree. For example, a review by the psyschologist John Loehlin in 1992 of five studies in five different countries (comprising a total sample size of 24,000 twins) estimates that genetic factors explain 60 per cent of the variance in extroversion and 50 per cent of the variance in neuroticism--magnitudes that are quite remarkable and that make clear that personality traits are highly heritable.
. . . A variety of studies have found heritability of this trait [aggression] in the range of 49 to 70 per cent . . . . Magnitudes of this size, across many different empirical studies, cannot plausibly be dismissed as experimental artefacts or the products of error. More importantly, they lend support to the view that Nietzsche strongly endorsed: "It is simply not possible that a human being should not have the qualities and preferences of his parents and ancestors in his body whatever appearances may suggest to the contrary" (Beyond Good and Evil). . . . "Wherever a cardinal problem is at stake, there speaks an unchangeable 'this is I'".
I lack the expertise, and desire, to read and evaluate the studies cited, for their experimental design and data analysis. My talks with WL have impressed on me the trickiness of deriving truth claims from data sets. Psychologists, unlike, say, economists, do not seem to rank high in the regard of hard statisticians. More truthful to me, by some non-empirical measure, is Leiter's understanding of Nietzsche's purpose in speculating about human psychology:
Any reader of Nietzsche knows that he is not primarily concerned to report the findings of psychological "research" or to establish, through conventional methods of argymentation and mustering of evidence, the truth of particular empirical hypotheses. His work is suffused with psychological and empirical claims, but his aims are always much more polemical: to transform the consciousness of at least some readers about the morality they take for granted, and thus, at the same time, to change their affective orientation towards their lives.
The last statement is most puzzling. For, if free will is an illusion and personality traits are largely inherited, how could any reader, even when challenged by the most brilliant polemic, change his affective orientation towards his life? How could the unchangeable 'this is I' change? If this is at all possible, it must be by changing the physiological make-up of the reader. Polemic cannot do that, since it appeals to the mind. Perhaps literature can. Emily Dickinson's test for poetry.