Hundreds of papers and books have been written, in which the authors are trying to prove by data on social mobility that human society is on the march to greater social equality. Most of these authors are not aware that their results are a function of their scaling of inequality. In order to measure social mobility you have to scale wealth, overall income, years of education, taxable income, social status or other appropriate variables none of which remained constant in the course of history. Because of random effects and imperfect scaling all these studies tend to overestimate intergenerational mobility. Already some researchers, who tried to scale recent and historical professions, jobs and social status according to underlying general intelligence to be successful, concluded that movements on the social ladder had changed little over the past centuries. This was substantiated by two books using samples of representative genealogical data covering several generations: La societe francaise au XIXe siecle: Tradition, transition, transformations (French Edition) and:Bevoelkerung Und Soziale Mobilitaet in Sachsen 1550-1880 (German Edition).
To measure social mobility in quite different countries and across centuries, Clark invented a novel technique: Tracking the frequency of surnames. Needed for such an approach are always data on the frequency of surnames in the general population and in the selected sample in the past and in the present. In a number of countries Clark and his coworkers were able to overcome these difficulties and to find or generate the databases necessary. The originality of this research deserves high praise.
However, to use surnames in such a way is not as new as Clark believes. About 1940 Karl Valentin Müller used frequencies of surnames of Czech and German origin to investigate their contribution to the upper stratum of cities in Bohemia. - Crow, J. F. and A. P. Mange published: Measurement of inbreeding from the frequency of marriages between persons of the same surname. Eugenics Quarterly 12 (1965) 199-203. Crow and Mange founded with this seminal paper a new branch of population genetics. Surnames can be understood as alleles of one genetic locus, and surname distribution and evolution can be analyzed by the theory of neutral mutations in finite populations. One may describe the genetic structure of a human population in terms of the inbreeding within its subpopulations and the extent of the sharing of genes among them. In the following decades, instead using marriage data, surname frequencies were also extracted from directories or census data. By applying these methods, the application of surname genetics was extended to measure genetic distance and historical changes within subpopulations and social strata, see, for example: Inbreeding and genetic distance between hierarchically structured populations measured by surname frequencies. Mankind Quarterly 21 (1980). And for an even wider outlook see: Familiennamenhäufigkeiten in Vergangenheit und Gegenwart als Ausgangspunkt für interdisziplinäre Forschungen von Linguisten, Historikern, Soziologen, Geographen und Humangenetikern. Namenkundliche Informationen 31 (1977) 27-32. However, 30 years ago, the databases for such an empirical approach were still lacking.
Outgoing from the medieval practice of giving surnames based on ones profession Günther Bäumler suggested a genetic-social theory of assortative distribution of traits of body build such as height, weight, and stature in a population of men called `Smith' (German: Schmied) and`Tailor' (German: Schneider). From this the hypothesis was deduced that among the top ranking athletes of the `heavy weight' branches of athletics, which require body strength and body height, there are relatively more persons that go by the name of Schmied than in the `light weight' branches of athletics, where more persons go by the name of Schneider. The hypothesis was empirically supported. See: Psychology Science 45 (2003) 254-262.
In the modern world we have a general negative relationship between the number of surviving children and the social status of their parents, in sharp contrast to the preindustrial world, where more children of the rich survive. Oded Galor and Moav Omer in their paper "Natural Selection and the Origin of Economic Growth" (2002) came to the conclusion that before 1850 the upper and medium stratum of society must have been more surviving children than the poor. Indeed, as a byproduct of his research with rare surnames Clark confirms that this turning point in differential fertility was in England already about 1850 (in Germany three or four decades later). Despite Clarkes conclusion that the most probable variable underlying social status and hence social mobility is the inheritance of general cognitive ability he dares not to cite the book IQ and the Wealth of Nations, supporting in such a way his argument on a global scale.
On some pages Clark seems to foster the belief that regression to the mean is a force equalizing any society in the long run. On other pages he is stating clearly that at the same time the random counterforce of segregation of genes is always creating new inequality in each new generation. Genetically pure lineages regress only to the mean of the line and not to the mean of the overall population. It is possible not only to study the decay of a social upper stratum by surname frequencies, but also its rise and creation in the course of some generations. In 1869 Francis Galton was the first to replace mere speculation on the inheritance of talent with statistical data. 100 highly gifted and very successful men had 26 fathers, 47 brothers, 60 sons. 14 grandfathers, 16 uncles, 23 nephews, 14 grandsons, 5 uncles of parents and 16 first cousins with similar giftedness and accomplishments. Astounding similar frequencies were found in other studies in different countries.
One can be sure that Clark will find followers studying the distribution and frequencies of French, Dutch, German and other surnames in the respective countries.