WASHINGTON — In recent decades, economists have been struggling to make use of the concept of human capital, often defined as the abilities, skills, knowledge and dispositions that make for economic success. Yet those who use the term often assume that to conceptualize a phenomenon is a first step to manipulating it. And, indeed, "human-capital policy" is now much in fashion. But what if many of the abilities and dispositions in question are a product of history, capable of being understood and explained but not readily replicated?