The fact that Clark is measuring something broader—or, to put it the other way around, that most economists are measuring only one element of overall socioeconomic status—helps explain why he estimates mobility to be so much slower. What’s actually being measured, by Clark and everyone else, is not mobility, but its opposite: the degree of persistence (in income, say) from parent to child. Each particular element of what makes up Clark’s “general social competence or ability” is of course subject to random variation. Any one individual, like the son-of-a-zillionaire philanthropist, may have an unusually small income but be a Princeton alum and belong to all the right clubs anyway. Conversely, a parvenu earning a bundle may have attended State Tech and be unable to join anything more exclusive than the local Y. But muddying up a statistical relationship with lots of purely random noise inevitably lowers the estimate of whatever is the object of interest, and so looking only at income (or education, or any other single feature) means finding less parent-to-child persistence. And that in turn means concluding, as many economists have done, that there is more mobility. By examining instead the whole array of attributes that constitutes socioeconomic status, Clark averages out the purely chance variation in any one factor, like income, and therefore finds greater parent-to-child persistence—in other words, less mobility.

But how does Clark get at the persistence of his all-encompassing concept of status? His method relies on the tendency, in most societies, for a son to bear his father’s surname (hence his book’s title: The Son Also Rises). Specifically, Clark measures the persistence of status by looking at what has happened over time to groups of people bearing names that, at some point in the past—generations or even centuries ago—indicated socioeconomic status either well above or well below that of the general population.

It’s fascinating how many such groups he has managed to round up. In America, his examples include people with the same last names as those who attended Ivy League universities in the early 19th century and people with identifiably Jewish names (in both cases groups with higher-than-average status); he also looks at those with names that suggest people who are black, or Native American, or the descendants of immigrants from French Canada (all lower than average). How he measures these groups’ socioeconomic status over time is quite specific as well: their educational attainment, their representation in elite professions like medicine and law, and, conversely, their representation in generally low-status occupations like farming and domestic service.

Clark applies the same method elsewhere, too. In Britain, some of the high-status groups he examines bear the names of families who arrived there with the Norman Conquest, or the names of families who owned enough land to have had their property officially recorded in the 1200s, as well as people whose last names correspond to specific geographic places. Here his subsequent measures of status include attending Oxford or Cambridge, having enough wealth at the time of death to be listed in probate records, and membership in parliament. In Asia, his high-status examples include Japanese whose names suggest that they are descendants of Meiji-era nobility or samurai, and Chinese whose names suggest that their ancestors earned top scores on the imperial examinations during the Qing dynasty. For these groups, his measures of status down the line include education, authorship of scholarly publications, and representation in elite professions or top government or business positions.