The said truth is that it is the greatest happiness of the greatest number that is the measure of right and wrong.

Jeremy Bentham (1748–1832)

The “greatest happiness principle” propounded by 19th century Utilitarian philosophers such as Jeremy Bentham and John Stuart-Mill directs that “right” actions are those which do the greatest good for the greatest number. Even if this philosophical principle has fallen into disrepute, having found its purest expression in the totalitarian excesses of fascism and communism, utilitarianism seems like a promising moral foundation, at least for public health medicine. However, medical research tends to be driven by the intellectual interests of individual researchers and rare diseases appear more often on grant applications than can be justified simply by the number of people affected. A commonly stated defence is that by understanding the pathophysiology of rare diseases we will learn something of importance about more common conditions. Are there good examples from neurology to support this approach?

Most genetic diseases are rare whereas most common diseases are considered to be “complex”, by …