Clark's provocative book draws on collaborative research which allows him to look at intergenerational social mobility across an impressive range of national and historical contexts, stretching across four continents and several centuries. His approach's key novelty is that it focuses upon surname categories, examining across a number of generations the persistence of the relative over- or under-achievement of various stratification-related outcomes by members of these categories. Clark observes a high level of consistency between his measures of social mobility for different times and places; his overarching thesis is that this reflects a relatively simple, universal ‘law’ of social mobility (p. 125). At the heart of his model is an underlying, unmeasured characteristic, sometimes labelled as ‘social competence’ or ‘social status’ (e.g. p. 11). However, he is inclined to conclude that this is determined and transmitted biologically (p. 14), although on occasions apparently conceding that cultural transmission is a possibility (e.g. p. 135), before subsequently attempting to undermine this (pp. 136–140).

Rather surprisingly, given that Clark is an economist, his argument views income and wealth merely as indicators of positions in the social structure, and of minimal relevance as intergenerational determinants of these (e.g. p. 107, p. 281). As a result of focusing on groups rather than individuals, the correlation coefficients that Clark uses to quantify (im)mobility differ substantially from the typical values obtained by economists examining income mobility (p. 9), creating a greater impression of persisting advantage (or disadvantage). While the techniques and conceptual ideas prevalent in sociological work on intergenerational mobility are largely absent, it would be a mistake to dismiss this book simply because it uses economic analyses of mobility as its main points of reference. Clark's arguments are frequently highly contentious, but the book is nevertheless a thought-provoking piece of work by an open-minded author, who views speculation as a legitimate way of prompting other academics to think about the mechanisms of social mobility (p. ix).

However, Clark's range of sources is necessarily opportunistic. While acknowledging that his grand thesis is based on incomplete evidence (p. ix), he seems less concerned about the risks of building a general theory of social mobility on data skewed towards elite outcomes. Given the current prominence of ‘big data’, Clark's book is a timely reminder that, however creative such uses of existing sources may be, they are shaped and constrained by the limitations of their data. Similarly, this book exemplifies the potential relevance of researchers’ measurements, models and assumptions to the results that they generate and any subsequent interpretations. Clark's model views educational attainment, occupation, income, etc. as indicators of a single, underlying ‘latent’ variable (p. 108), rather than components of an individual's social position viewed in broad terms (or multidimensionally). Clark's focus on this underlying ‘social status’ allows him, rather neatly, to disregard published findings relating to the effects of grandparents’ characteristics, interpreting them as reflecting the partial evidence about individuals and their parents provided by measures based on a single characteristic such as occupation (p. 119). From Clark's perspective, a stumbling block in the way of a genetically dominated explanation of social (im)mobility is thus removed.

It is not easy to assess the extent to which Clark's evidence and argument are shaped by an assumption that, within any particular surname group, underlying ‘social status’ has a normal distribution (e.g. p. 41, p. 56, p. 298), or whether this assumption predisposes the results towards consistency with the importance of genetically determined ability. More generally, Clark's model is arguably too heavily influenced by economists’ predilection for interval-level measurement, perhaps reinforced by a greater familiarity with North American socio-economic status (SES) scales than with the class schemata often utilised in European sociology. Similarly, much of Clark's evidence (e.g. for the United States: pp. 45–69) relates to particular professions as outcomes; greater familiarity with analyses of occupational mobility tables might have alerted him to the salience of the more specific mechanisms underpinning exact occupational inheritance, over and above those determining patterns of mobility more generally.

In some respects Clark's finding of consistency over time resonates with the ‘constant social fluidity’ model often used as a point of reference by Goldthorpe and other sociologists such as Li and Devine. However, while Clark suggests (quite reasonably) that some of the apparent variations between his correlations are attributable to sampling error (e.g. p. 96), I was left wondering whether some of his sets of correlations exhibit sufficient diversity to allow for substantively important deviations from his ‘law’ of social mobility which he does not acknowledge (p. 117). Furthermore, the properties of correlations are not identical to those of other measures of inequality; the tendency for competing measures such as odds ratios and Gini coefficients to identify different trends over time, especially when there are changes in the likelihood of advantageous outcomes, highlights the possibility that the consistency observed by Clark may sometimes be an artefact of his use of correlations. Similarly, the ‘relative representation’ measure that Clark often uses for descriptive purposes (e.g. pp. 31–40) is a contentious choice when the distribution of outcomes is altering over time.

Ironically, Clark's analytical leaning towards patterns of inheritance being driven by an underlying factor leaves little room for names to have a direct effect on outcomes. This seems counter-intuitive, although not to the same degree as his suggestion that material resources have no causal impact. In the UK, social intervention to promote mobility is a high profile contemporary issue, and parents continue to strive to enhance the outcomes for their children. On the basis of his model, Clark is sceptical about the extent to which either governments or parents can achieve their objectives (pp. 268–274; p. 281). He instead advocates a less fashionable objective: aiming for greater equality in the outcomes and rewards accruing to different social positions (p. 275).

Clark suggests that intervention may not, in fact, be needed to achieve meritocracy, as his model does not rule out the possibility that it already exists, while providing no evidence that it does (p. 262)! He does find room for human agency to have a (limited) impact (pp. 262–263), but those who manage to ‘over-achieve’ relative to their underlying ‘social competence’ will in general, according to Clark, not be able to use this to enhance their children's social positions (e.g. p. 125). His argument implies one exception to this: where over-achievement allows someone to (biologically) co-parent children with a partner whose underlying ‘social competence’ is greater than that of the type of partner that the over-achiever would otherwise have acquired (p. 15). Thus, just as Clark's book implicitly highlights the importance of considering more than one aspect of social position (before collapsing all such aspects into a single underlying factor!), it also highlights the contribution of homogamy to social (im)mobility, and hence the importance of taking both (biological) parents into consideration when looking at outcomes for children. This feature of the book is rather striking, given the virtual absence of discussions of gender, which is a reflection (in part) of its patronymic focus, as Clark acknowledges (pp. 15–16).

Sociologically at least, Clark's book appears relatively atheoretical, compared, for example, with another contentious contribution to the recent stratification literature: the Bourdieusian model of the class structure associated with the ‘Great British Class Survey’. However, these two contributions share a shift in emphasis: away from class conceptualised solely in occupational terms. This prompts an interesting question: might the consistent patterns observed by Clark, rather than reflecting inherited ‘social competence’, reflect instead the intergenerational transmission of ‘composite capital’, with this process being relatively unaffected by any variations in this capital's typical composition? This alternative explanation lacks the disadvantages of being unconvincingly static and implausibly biologically deterministic.

To reiterate, Clark's faith in his ‘law of social mobility’ rests upon the consistency that he sees as existing between his empirical evidence and a model which views social positions as primarily reflecting the impact of some biologically inherited underlying quality, supplemented by (near-enough) random ‘shocks’, the latter being transitory and lacking any intergenerational carry-over (pp. 107–125). The challenge for readers who smell a substantial rat is to identify some way in which the apparent consistency of Clark's model with his evidence is artefactual or misleading, or to identify a more complex model, with a less static and straightforwardly deterministic core, which is equally consistent with comparable evidence. Will sociologists, then, queue to pick up the gauntlet thrown down by Clark? Maybe not, as some may have smelt a rat from a distance, rather than open-mindedly reading his book. If so, that is arguably a shame.

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