Validity refers fundamentally to the extent to which statements can be regarded as valid or true in the light of empirical evidence (Shadish et al., 2002, p. 34). A measurement can be considered valid if a concept is captured in its entirety, is clearly delimited from adjacent concepts and if the measurement is as unbiased as possible.

Following Bourdieu, cultural capital is a multidimensional concept. He distinguishes between three forms of cultural capital: institutionalized, incorporated, and objectified cultural capital (Bourdieu, 1986, pp. 243–247). The term institutionalized cultural capital refers to publicly recognized and institutionalized educational certificates. These certificates are convertible into professional positions on the labor market and, thus, different incomes (economic capital). Incorporated cultural capital refers to embodied skills and knowledge acquired during the processes of family and school socialization. Finally, objectified cultural capital comprises material goods to which a cultural value is attributed, such as artworks or books.

Although Bourdieu’s concept of cultural capital is very popular in research, it is not without criticism. It has been pointed out that the definition of legitimate cultural capital is highly context-specific (Lamont & Lareau, 1988) and that the mechanisms of intergenerational transmission remain unclear (Jæger, 2022, p. 122; van Hek & Kraaykamp, 2015; Lareau, 2003).

At the measurement level, there are numerous indicators attempting to capture the various forms of cultural capital. Typically, educational certificates or the number of years of schooling are used to measure institutionalized cultural capital (e.g., Rössel & Bromberger, 2009). Incorporated cultural capital is often measured via cultural practices assumed to be associated with the acquisition of cultural knowledge, such as attendance of cultural events outside the home or practices within the home, such as reading (e.g., Börjesson et al., 2016; Kraaykamp & van Eijck, 2010). Objectified cultural capital is captured through the measurement of cultural possessions. While the number of books at home is frequently used to measure objectified cultural capital (Sieben & Lechner, 2019, p. 2), some research captures cultural possessions more comprehensively (e.g., antique furniture and music instruments; Kraaykamp & van Eijck, 2010).

The number of books at home, however, is one of the most, or even the most commonly used indicator in research on educational inequality (e.g., Hanushek & Woessmann, 2011, p. 117) and has been shown to correlate positively with other indicators of cultural capital (e.g., Sikora et al., 2019; Evans et al., 2010; Goßmann, 2018). The indicator’s prominence can be partly explained by its parsimonious measurement, as it only takes one item to be included in a questionnaire. Another factor contributing to the indicator’s pervasiveness is its broad conceptual meaning. It serves as a proxy for the total volume of cultural capital in a household as well as for the conditions of cultural socialization of children living in a household. It is further a proxy for domain-specific cultural capital in the literary field. This broad meaning of the indicator is a strength, as it can be considered a catch-all solution for the aforementioned concepts. But, at the same time, it implies a conceptual weakness (Jæger, 2022, p. 122; van Hek & Kraaykamp, 2015; Lareau, 2003) and raises the question what the number of books in a household is a proxy for (Engzell, 2021). In a paper published in Measurement Instruments for the Social Sciences, Sieben and Lechner (2019) assess the validity of the indicator “number of books in the household”. They conclude that it is valid for capturing the objectified cultural capital but should not be considered an indicator for cultural capital in general, as they find only small correlations of the indicator with cultural and literary activities.

With the digital transformation of society, the question arises as to how valid the traditional indicator of the number of printed books at home is in the context of the mass distribution of e-books. Between 2011 and 2021 the proportion of Americans who say that they have read an e-book in the past 12 months rose from 17% to 30% (PEW Research Center, 2022). A similar trend can be observed in Germany, where the share of e-book sales in total book sales rose from 0.5% in 2010 to 6.0% in 2022. However, it should be noted that e-book sales have stagnated since 2020 and the number of e-book buyers even decreased between 2013 (3.4 million) and 2022 (3.0 million). It is mainly the younger people who read e-books (Börsenverein des Deutschen Buchhandels, 2023). This last finding is in line with the literature on the diffusion of innovations (Rogers, 2003) and the digital divide (van Dijk, 2005). Although e-books are sold more cheaply than printed books or are even available for free, this literature suggests that particularly younger and more educated individuals make use of digital technologies. If some social groups digitize their reading habits while others do not or only to a lesser extent, a validity problem arises for the traditional indicator of the number of books at home: We then underestimate the objectified cultural capital of some groups. Accordingly, we address the question of how valid the traditional indicator “number of books at home” is in times of digitization and investigate whether an indicator that combines digital and printed collections yields substantial advantages over the traditional indicator.

We do so by distinguishing between the convergent and the discriminant validity (Campbell & Fiske, 1959). A concept has convergent validity if there are high correlations between measures of the same construct. In our case, this implies positive correlations with other literary practices. A concept has discriminant validity if measures of the same construct correlate stronger with each other than with measures of a different construct. Books contain written knowledge and make it possible to pass over knowledge of all kinds. Hence, we can speak of discriminant validity if correlations with practices in other cultural fields are less strong than with practices in the literary field.

We first present descriptive findings on the distribution of book collections in physical and digital form (Table 1, Figure 1) and the correlation of a combined indicator with physical and digital collections (Figure 2). Correlations of physical, digital, and combined book collection with practices in the literary field (Figure 3), institutionalized cultural capital, and cultural socialization (convergent validity, Figure 4) as well as practices in another cultural domain (discriminant validity, Figure 5) are then presented. In each case, the correlations are reported for the full sample and for three age groups. We distinguish between three age cohorts, as age is a key predictor of participation in digital information and communication technologies (ICT) in the literature on the digital divide (e.g., Sawert & Tuppat, 2020). While the youngest group, 15 to 35-year-olds, have grown up with ICT as digital natives, this does not apply to the oldest group, over 55-year-olds. These are often referred to in the literature as digital immigrants, as they have to actively adapt to the new technologies (e.g., Ballano et al., 2014).

As digital transformation is a dynamic phenomenon, we conducted a re-analysis based on the refreshment sample from the second wave from 2021 to validate our findings. For several reasons, we only used the data from 2021 for the re-analysis and not for the main analysis. One reason was that activities outside the home were greatly restricted due to the coronavirus pandemic. Another is that some variables were no longer collected in 2021. Finally, the sample size of the 2021 wave is substantially smaller than in 2018. The re-analysis with the more recent data, which is documented in Appendix B (see Balzer et al., 2025), does not come to any different conclusions.

Using survey data of the German population from 2018, we investigated whether we are missing something when using book collections as a measure of cultural capital without taking digital collections into account. We found that digital book collections do not yet play a formative role compared to print-collections. Only just under 30 percent of the population aged 15 and older owns e-books. More relevant, however, is the question of whether those people who own e-books replaced their physical collection with them. If this was true, we would systematically underestimate the size of the collection for some groups. Our finding indicates that this does not seem to be the case. It is also important to point out that the size of a person’s e-book collection is considerably less correlated with his or her reading frequency than that of the printed book collection, even for the youngest age group. This observation casts doubt on the convergent validity of the digital indicator. When the digital collection is considered in addition to the physical collection, the information gains are marginal empirically. This finding is in line with Heppt et al. (2022), who show that the e-book collection does not have any additional explanatory power beyond the physical book collection in explaining children's academic achievement.

We used data from a panel wave conducted in 2018, as they allow for the most comprehensive analyses of convergent and discriminant validity. As the data used in our analyses is already seven years old, we have repeated the analyses using more recent data from 2021. The results are not substantially different. Hence, we are optimistic that our main conclusions are meaningful. Nevertheless, it must be taken into account that the digital transformation is a dynamic process, and it is possible that a similar study will come to different results in five or ten years. This time restriction is supplemented by a spatial reservation. Compared to the USA, for example, e-books are comparatively less widespread in Germany. The question is to what extent the findings also apply to countries in which the digital transformation of society is more advanced. Although the literature on the digital divide and the diffusion of innovations suggests that social gradients should become weaker as mass adoption increases, whether this is confirmed empirically is a yet open question.

Although our study is only a temporal and spatial snapshot, we currently see no problem in only taking the number of physical books into account. However, we have mentioned literature arguing that the indicator is conceptually imprecise. According to our findings on convergent validity, it can be recommended as a rough indicator to record socio-cultural background. However, if one is interested in causal studies that investigate specific mechanisms, the indicator seems less suitable.

Our practical implication is that currently e-books only need to be measured for research questions about digital behavior and are not necessary for researching general questions on intergenerational (im)mobility. At the same time, we call for the validity of this conclusion to be examined regularly as digital transformation progresses.