online book purchases reveal partisan science interests

Written by Feng Shi, Yongren Shi et al.

Abstract: Passionate disagreements about climate change, stem cell research and evolution raise concerns that science has become a new battlefield in the culture wars. We used data derived from millions of online co-purchases as a behavioural indicator for whether shared interest in science bridges political differences or selective attention reinforces existing divisions.

Findings reveal partisan preferences both within and across scientific disciplines. Across fields, customers for liberal or ‘blue’ political books prefer basic science (for example, physics, astronomy and zoology), whereas conservative or ‘red’ customers prefer applied and commercial science (for example, criminology, medicine and geophysics).

Within disciplines, ‘red’ books tend to be co-purchased with a narrower subset of science books on the periphery of the discipline. We conclude that the political left and right share an interest in science in general, but not science in particular. This underscores the need for research into remedies that can attenuate selective exposure to ‘convenient truth’, renew the capacity for science to inform political debate and temper partisan passions.

In its quest for an objective understanding of the world1, modern science has practised two distinct forms of political neutrality: as an apolitical ‘separate sphere’ detached from ideological debates, and as a ‘public sphere’ relevant to political issues but with balanced political engagement that aids reasoned deliberation and deference to evidence2,​3,​4,​5.

Recent surveys support the view that science contributes not only to human knowledge but also to social integration, both as a voice of reason and also as a shared value. Joint surveys conducted by the American Association for the Advancement of Science (AAAS) and the Pew Research Center in 2009 and 2014 found that science remains near the top in public rankings of professions, well above that of clergy, despite the prevalence of liberals among scientists6,​7,​8.

Although nearly two-thirds of respondents question evolution, even those who see conflict with issues of personal faith overwhelmingly support scientific contributions to public well-being (67%). In a highly polarized electorate, these responses invite reassurance that science continues to command political deference as a voice of reason and bridges the partisan divide. We may disagree on emotionally charged social issues, but at least we can agree on science.

Political and cultural polarization within the United States, however, raises questions about the validity of this interpretation9. A less comforting possibility is that verbal survey responses may simply echo an Enlightenment commitment to value-free scientific inquiry that masks underlying scepticism about science. In recent years, conservative politicians and pundits have challenged scientific positions on evolution, cosmology, climate change and the perceived liberal bias in policies advocated by social scientists. For example, the conservative-funded scientific counter-movement in climate change research suggests the possibility of politically driven scientific polarization10,11. When science becomes politicized, partisans tend to cast doubt on scientific consensus through questioning its inherent uncertainty12,​13,​14.This process is manifest not only in conservative resistance to climate change, but in historically liberal resistance to consensus over the positive benefits of genetically modified organisms, vaccination, nuclear power and the safe storage of nuclear waste15.

Survey data show little overall change in public confidence in science since 1970, but beneath the surface there is a marked shift: conservatives in the Vietnam era were more confident in science than liberals, but today that pattern has reversed16 (Supplementary Fig. 1). Does public exposure to science play an integrative role by encouraging and informing empirical validation? Or has selective attention instead reinforced the ‘Big Sort’ of American politics17,18,19— the tendency to cluster in like-minded communities?

Much previous research has used surveys to investigate political alignments of the producers of science (with a few exceptions20,21). We focus instead on the consumers of science, using online co-purchases of books on science and politics as a behavioural indication of preferences held by customers who ‘vote with their pocket­book’, in contrast to survey responses that are costless. Surveys measure what researchers think is important, not what respondents care about, whereas online consumers can register their preferences by purchasing books on any topic they choose.

Retrospective self-reports are vulnerable to lapses of memory, whereas online sellers track every purchase. Survey responses are difficult to align across instruments that ask different questions and ask questions differently, whereas books from different stores can be classified using consistent typologies (for example the Library of Congress). Surveys are vulnerable to response bias from participants reluctant to reveal views regarded as politically incorrect; books purchased online arrive cloaked in cardboard.

Finally, although surveys can use stratified random samples to generalize results to the underlying population, which is not possible with data from a convenience sample, rates of non-response are rising in landline-administered surveys, which raises concerns about their external validity22.

We addressed concerns about generalizability in two ways — by replicating our analysis using two independent samples of purchasing behaviour from two online merchants (Amazon and Barnes & Noble), and also by the size of these samples, collectively comprising hundreds of millions of online customers, including members of hidden populations (such as those without landlines) who may be undercounted in surveys based on at most a few thousand respondents.

Our approach also differs from survey methods in the unit of analysis. Individuals are the units in surveys, but online retailers do not provide access to individual customer behaviour. Instead, we use individual books as the unit of analysis in constructing a co-purchase network. Bipartite network analysis has been widely used in research on co-citation and co-author networks23,​24,​25,​26,​27 where individual information is aggregated to provide connections between scientific papers.

What is lost in the absence of individual-level responses for a small sample is compensated for with massive numbers of observations we can use to detect population-level patterns generated by individual behaviour. Here we extend bipartite network analysis to the study of online co-purchase behaviour among a diverse population of tens of millions of consumers. These data provide an unprecedented opportunity to study the entire popular audience for science in ways that are not possible using traditional methods.

These data do not speak to the partisan alignment of scientists, the policy relevance of scientific research or the political polarization of science as an institution. Nor do we address the political preferences of science’s consumers. Rather, our attention is focused exclusively on the science preferences of those who purchase liberal and conservative political books, a group whose science preferences could differ from those who do not shop for books online or who shop for science books but not for politics. Within that constraint of available data, we ask to what extent purchasers of political books are also interested in science, and in what parts of science they are most interested. A shared interest in science might provide a bridge across partisan divisions, whereas selective attention to ‘convenient truths’ risks reinforcement of existing political identities.

To find out, we constructed two undirected co-purchase networks of books from the American domain of the world’s two largest online book stores, following an approach pioneered by Valdis Krebs28,​29,​30. Up to 100 unranked books are listed on each book page under the heading “Customers Who Bought This Item Also Bought”. According to previous work31, these recommendations are based on a disproportionate number of customers who bought both books and are designed exclusively to identify those books (up to 100) that a customer is most likely to be interested in also purchasing. We compared results derived from two different bookstores — Amazon, and Barnes & Noble — each with its own co-purchasing algorithm. No important differences were found (see Supplementary Information).

Differences in the relative popularity of two books A and B could cause B to be included in the top 100 list for A while A does not make it into the list for B. Co-purchase behaviour is inherently undirected at the level of an individual customer’s multiple purchases, however, as the purchase of book A cannot be said to cause an individual to purchase B any more than the converse. We therefore ignore direction and reciprocation and define an undirected co-purchase link between two books as the level of bi-directed co-purchasing required to trigger a co-purchase listing in either direction.

Beginning with several seed books, we collected data recursively by tracing co-purchase links, iterating the search until no new titles could be identified. In total, we collected 26,467,385 co-purchase links among 1,303,504 books from amazon.com after consolidating multiple editions. (For details, see Methods. These counts refer to the number of titles, not purchases; we use ‘books’ to refer to titles and never to the physical objects.) From this collection, we identified three groups: political books, science books, and non-science books. From 3,530 politically relevant titles, two independent coders (with a third as tie-breaker) identified 673 conservative (‘red’) books, 583 liberal (‘blue’) books, and discarded 2,274 indeterminate books. The ‘indeterminate’ books include some that are centrist, and these could be used to investigate different interests in science held by moderates and those with more extreme views (whether red or blue). That possibility falls outside the scope of the present study, which is limited to comparisons of red and blue co-purchases.

As an additional validation, we imputed red and blue codings based on the relative number of links to other red and blue books and compared these with the hand codings. Over 96% were in agreement. The network among blue and red books is visualized in Fig. 1a. The monochromatic clustering reveals the political ‘echo chambers’ in a highly polarized population. (See Methods for methodological details, red/blue book comparisons that show similarity in publication year and sales rank, and information about the handful of ‘misfits’ in each cluster.)

Figure 1: Visualization of the co-purchase network among liberal, conservative and scientific books.

Figure 1

a, Links between 583 liberal (blue) and 673 conservative (red) books. b, Links between these books and science (grey) books. As shown in a, 97.2% of red books linked to other reds and 93.7% of blue books linked to other blues. A small number of books were more likely to be co-purchased with books of a different colour. We subjected these blinded books to additional judges and found that the original codings were nearly all correct. A number of red ‘orphans’ were written by moderate Republicans critical of the religious right while blue ‘orphans’ were written by progressive community organizers such as Saul Alinsky (Rules for Radicals40), later rediscovered by the Tea Party who reference their effective ethnic blue-collar organizing tactics. In b, the broader distribution of blue-linked science books indicates that readers of blue books have broader disciplinary interest and co-purchased books more centrally located in the network of co-purchased science books.

The political and science books identified in amazon.com were also collected from barnesandnoble.com, to test the consistency of co-purchasing patterns between the two bookstores. These co-purchase networks, composed of millions of distinct purchases, are different in important ways: only 9% of Amazon co-purchase links are found in the Barnes & Noble network, and only 21% of Barnes & Noble links are found in Amazon. Nevertheless, the number of political links with each book in Amazon and Barnes & Noble is highly correlated (0.60). The consistency of our findings in these two environments suggests robust patterns of co-purchase behaviour, in which the political preferences of science book consumers are very similar even though the expression of those preferences in the purchase of particular books differs across the two websites.

We identified 428,433 titles that appeared under science categories in the Library of Congress and Dewey Decimal classifications. We grouped these into 27 exclusive high-level topics, corresponding to broadly defined scientific disciplines (such as physics, chemistry, medicine or economics). These 27 disciplines fall under four main scientific ‘schools’ (humanities, physical sciences, life sciences and social sciences). An additional 494,278 non-science titles were grouped in four main topics — arts, sports, literature (fiction and poetry), and religion — as a baseline for assessing co-purchase links between science and politics (see Methods for detailed categories).

We used co-purchase links to measure the political relevance, alignment and polarization among online customers. Political relevance of a topic is measured by the fraction of links from political books (whether red or blue) among all co-purchase links to books in the topic. Given the measure’s uncertainty with small samples, we used a Bayesian probabilistic framework with a prior distribution induced by the configuration model32. Links between science, non-science and political books are randomly generated, given the network degree of each book. The higher the political relevance, the greater the likelihood that purchasers of political books will purchase books in the topic.

Political alignment of a topic is measured by the fraction of links from red books among co-purchase links from political books (red or blue). As with relevance, we account for statistical uncertainty by estimating this measure as the probability that books, in a particular topic will link with red books, conditioned on their links from political books. Alignment captures partisan interest in each scientific discipline on the red–blue spectrum, where purple (alignment = 0.5) could indicate the balanced political interest required for a public sphere of reasoned discourse.

Alternatively, a ‘purple’ discipline could also be internally polarized, with equal interest from left and right but in separate subsets of books. Political polarization is a function of the number of books within a discipline linked with both red and blue books, compared with a null model in which red and blue links are randomly assigned to books in the topic. Polarization equals 0 when disciplinary books are co-purchased with red and blue books uniformly at random, but increases as the sets of red- and blue-linked books diverge, indicating red and blue preferences for distinct books. (Note that we use the term ‘polarization’ to refer to the political segregation of the population into opposing groups of like-minded members who influence one another to adopt similar positions on other issues on which they did not initially agree33. Our usage should not be confused with an alternative definition of polarization that indicates the tendency of people who agree on a given issue to influence one another to adopt more extreme opinions on that same issue.)

In addition to these three metrics, we also measured characteristics of scientific books and fields to account for diverging red and blue scientific interests. We scored fields as basic or applied science based on the ratio of patent to article citations in each field. We also considered whether books are ‘academic’ or ‘popular science’, based on publication by an academic or popular press. Finally, we measured the relative scientific breadth of liberals versus conservatives as the difference between the average number of disciplinary titles linked with a blue book and those with a red book, and the network location of books with ties to red and blue relative to the core or periphery of the co-purchase network within the disciplines. (See Methods for detailed discussion of these measures.)

Our analysis proceeds in three steps. We first assess political relevance, alignment and polarization as measures of political interest in science compared with political interest in books and topics outside science; second, we report differences in these measures across scientific disciplines, broken down by the continuum of applied and basic science; third, we report the scientific breadth and location of red and blue books within disciplines.

First, compared with the number of co-purchase links expected by chance, people who buy liberal and conservative books are more likely to buy books on science than on important topics outside science, but this interest in science does not appear to be insulated from the ‘Big Sort’ of American politics. Figure 2 reports political relevance and polarization of science, religion, sports, arts and literature, with alignment indicated by colour. Results for political relevance show that political readers have greater interest in science relative to non-science topics, largely owing to books on social science. The physical and life sciences do not attract markedly greater interest among political readers compared with topics outside science, but this political interest in science is significantly more polarized than for arts and sports, indicating that liberals and conservatives are less likely to read the same science book.

Figure 2: Comparisons of political relevance, polarization and alignment between science and non-science books.

Figure 2

Colour indicates alignment from liberal (blue) to conservative (red), corresponding to the relative interest in the topic by liberals versus conservatives. The widths of the bars (x axis) in the left panel indicate political relevance of topics of science and non-science books, and those in the right panel indicate their political polarization. Science books attract equal interest from liberals and conservatives but are more politically relevant and polarized than non-science books, largely owing to the social sciences and humanities, while the physical and life sciences are similar to non-science overall. Books on the performing arts are most aligned with liberals and, along with sports, have low political relevance and polarization compared with literature, religion and science.

Second, we found a significant positive correlation (r = 0.43, p = 0.002) between political alignment and the normalized number of citations by patents, our indicator of applied, commercial science (Fig. 3b). For example, organic chemistry is the most applied sub-discipline, as measured by patent versus article citations, and it aligns closely with red books (0.75 on an alignment scale from 0 to 1). This contrasts with a sub-discipline such as zoology, which is largely driven by curiosity and basic scientific concerns, and appeals more to those on the left (0.1). This pattern can also be observed in Fig. 3a, which reports co-purchase alignment within the four ‘schools’. Applied disciplines such as medicine and law attract readers at the red end compared with other disciplines in their respective schools, whereas anthropology and astronomy attract readers at the blue end. This mirrors the ideological differences among scientists employed in academia versus industry, as reported in AAAS/Pew surveys6,7 from 2009 and 2014. A possible interpretation is that scientific puzzles appeal more to the left, while problem-solving appeals more to the right.

Figure 3: Comparisons of political alignment across scientific disciplines and sub-disciplines.

Figure 3

a, Colours and values in the disciplinary tree indicate political alignment. Differences in alignment emerge more clearly the more disaggregated the categories. Books in applied disciplines such as medicine, law and climatology are more likely to be co-purchased with conservative books, whereas those in basic science disciplines such as zoology, anthropology and philosophy are more likely to be co-purchased with liberal books. b, This association becomes clearer at the sub-discipline level, revealed in the scatter plot. Conservative alignment tends to increase with the applied index (r = 0.43, p = 0.002), which is defined as the ratio of patent to article citations, here shown on disaggregated Dewey Decimal subfield data along with a 90% confidence interval.

A few disciplines, notably palaeontology, bridge political divisions and are politically purple because books in these disciplines attract equal interest from both left and right. However, most purple disciplines (with equal likelihood to have red and blue book links) do not bridge political divisions. Figure 4 illustrates the internal network structure of seven natural and social science disciplines. Monochromatic clusters, located in different regions of the network, indicate that red and blue political books are linked to different interconnected clusters of disciplinary books. Simply put, even when left and right are equally likely to read books in a discipline, they are rarely the same books or even from the same topical cluster.

Figure 4: Location of red and blue political books within the network of disciplinary books to which they are linked.

Figure 4

Network visualizations show the locations of red and blue political books in the co-purchase networks of science books (grey nodes) in seven disciplines: zoology, biology, environmental science, climatology, economics, political science and philosophy. All disciplines show high polarization in partisan co-purchasing, with red books linked to fewer disciplinary books and to disciplinary books closer to the periphery (Supplementary Fig. 4). The peripheral location means that these red books are co-purchased with disciplinary books that are less likely to be co-purchased with other books in the discipline. The monochromatic clustering is due entirely to links among red-linked science books and likewise for blue-linked science books; the links between political books are not considered in constructing the networks. The scatter plot shows that the more polarized the political co-purchases, the higher the relative scientific breadth of blue versus red, as measured by the difference between the average number of disciplinary titles linked with a blue book and the number linked with a red book (r = 0.8, p < 0.001). The line represents the estimated relationship, along with the 90% confidence interval.

We drilled down further to examine the location of science books linked with red and blue books within co-purchase networks at the disciplinary level. The scatter plot in Fig. 4 presents the relative scientific breadth of blue versus red (y axis). Note that higher values in the y axis indicate greater difference between the breadths of blue and red books. The figure reveals that blue books link to a larger number of scientific titles than red within most disciplines, and the difference increases for more polarized disciplines.

The disciplinary networks show the location of red and blue political books linked to disciplinary books within a field (coloured grey). All books are positioned such that pairwise proximities correspond only to co-purchase links with disciplinary books. The visualizations show that red books tend to cluster on the periphery of the co-purchase networks for climatology, environmental science, political science and biology, which indicates that conservatives tend to purchase books likely to be co-purchased with each other but not with other books in the discipline (Supplementary Fig. 4). In contrast, blue books are less clustered and linked to books closer to the core, which indicates that liberals tend to purchase a more diverse set of science books, including books frequently co-purchased with other books in the discipline. The greater centrality of blue-linked books does not appear to be a consequence of academic liberalism. When books by academic publishers are removed, the pattern remains, as do all the other patterns that we report (see Supplementary Fig. 5).

These results need to be qualified by inherent limitations in the use of co-purchase links to measure partisan interest in science. First, although half of the US population purchases books online, this is not a random sample, which limits the ability to generalize our results to the other half. We cannot be certain that those who purchase books online are similar to those who do not, or that those who purchase red and blue books have the same interests in science as liberals and conservatives who do not purchase political books.

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