Doctrinal development and the importance of judicial opinions

Opinion writing is central to judicial decision-making. Hazelton and Hinkle note in their recent work that, “[i]t is the majority opinions, not outcomes, that lower courts and government officials interpret and implement” (Reference Hazelton and Hinkle2022, 157). Perhaps most significantly, written opinions clarify the law for the parties involved in any particular case and, by extension, serve as a source of transparency for other judicial actors, governmental agencies, and the public at large (Corley, Collins, and Calvin Reference Corley, Collins and Calvin2011). Written opinions are critical in establishing and refining relevant precedents that inform how the law is adjudicated in subsequent cases throughout each tier of the judicial hierarchy (Callander and Clark Reference Callander and Clark2017).

Studies examining judicial opinion content often focus on the practice of language borrowing, where high court judges and justices directly implement language from lower court opinions into their own decisions, thus codifying the language from the lower courts as national precedent (Corley, Collins, and Calvin Reference Corley, Collins and Calvin2011). There is considerable evidence that higher courts routinely borrow language from lower court opinions because these opinions provide information on the state of the law as it relates to the case at hand, offer persuasive legal arguments, and cover complex issues (Corley, Collins, and Calvin Reference Corley, Collins and Calvin2011; Savchak and Bowie Reference Savchak and Bowie2016; Bowie and Savchak Reference Bowie and Savchak2022). Other work on bottom-up influences reveals that Supreme Court justices ascertain critical information on the applicability and policy consequences of legal rules through implementation patterns within the lower courts (Hansford, Spriggs, and Stenger Reference Hansford, Spriggs and Stenger2013). In addition, research by Clark and Carrubba (Reference Clark and Carrubba2012) and Carruba and Clark (Reference Carrubba and Clark2012) suggests that Supreme Court justices utilize the lower courts as a means to mitigate the substantial costs associated with developing new doctrine (see also Beim Reference Beim2017). Beyond bottom-up accounts, studies demonstrate that justices are not averse to turning to external sources to help bolster the content of their opinions (Corley Reference Corley2008; Collins, Corley, and Hamner Reference Collins, Corley and Hamner2015; Black, Owens, and Brookhart Reference Black, Owens and Brookhart2016). Given the evidence in support of opinion content being shaped by dynamic factors, studying language borrowing tendencies between courts helps provide key insight into how justices craft their opinions and the ways in which lower court judges help shape the development of legal doctrine from the bottom up. In the next section, we explore the factors that influence language borrowing tendencies among Supreme Court justices and lower court opinions in the United Kingdom.

The institutional dynamics of the UK judiciary

The Supreme Court of the United Kingdom, which began operations in 2009, was created by the Constitutional Reform Act of 2005. The new UK Supreme Court replaced the Appellate Committee of the House of Lords to provide greater judicial independence from Parliament and to further distinguish the functions of the Court as inherently nonpolitical (Hanretty Reference Hanretty2020). The UK Supreme Court hears appeals from the three distinct judicial systems – England and Wales, Scotland, and Northern IrelandFootnote ⁴ – that make up the UK legal structure (see Hanretty Reference Hanretty2020, 6–7). Most cases begin in the High Court (i.e., trial court of first instance) and are heard by a single judge. Decisions made by the High Court may be appealed to the Court of Appeal, which is the principal intermediate appellate court in the UK. The Court of Appeal of England and Wales is made up of the Criminal and Civil divisions, where cases are decided by three-judge panels (Blanes i Vidal and Leaver Reference Blanes i Vidal and Leaver2013; Hanretty Reference Hanretty2020). Unless the UK Supreme Court decides to review a case on appeal, the Court of Appeal of England and Wales is effectively the final arbiter of most judicial appeals.

The mechanisms that help explain language borrowing in the United Kingdom can be attributed to the judiciary’s unique institutional structure and professional norms. Unlike other common law judiciaries in which the appointment process is political, the UK system is a career-based judiciary, meaning that judicial advancement is largely predicated on prior record of service and a judge’s reputation among his or her judicial peers (Liu and Zhang Reference Lui and Zhang2001). To be appointed to the Court of Appeal of England and Wales, candidates not only have to be recommended to the monarchFootnote ⁵ by the Prime Minister on behalf of the Lord Chancellor, but they are also required to have a combined twenty-five years of experience serving as barristers or advocates of the court (Hanretty Reference Hanretty2015; Masood and Lineberger Reference Masood and Lineberger2020). Such substantial tenure requirements for judicial advancement distinguish the UK courts from most other legal systems where judicial appointments are, at least in part, political in nature (Blom-Cooper, Dickson, and Drewry Reference Blom-Cooper, Dickson and Drewry2009).

Advancement to the Supreme Court is also based on experience and expertise. Candidates for elevation to one of the twelve positions on the Court must have either served on a High CourtFootnote ⁶ or the Court of Appeals for at least two years or have practiced law for a minimum of fifteen years. After the Judicial Appointments CommissionFootnote ⁷ reviews each candidate’s application, the committee’s nominee must be approved by the Law Lord Chancellor, the Prime Minister’s Office, and, ultimately, the monarch. Importantly, nominees cannot be an active member of a political party and justices must retire at the age of 70.Footnote ⁸

Given the apolitical and merit-based nature of the appointment process, the structure of the UK judiciary incentivizes lower court judges to craft opinions grounded in legal precedent rather than motivated by ideological considerations (Atkins Reference Atkins1990; Salzberger and Fenn Reference Salzberger and Fenn1999; Malleson and Moules Reference Malleson and Moules2010; Darbyshire Reference Darbyshire2011). Decisions that are based largely on precedential and legal factors enhance the legitimacy of judicial opinions and, in turn, increase the likelihood of further promotion up the judicial ladder (Drewry, Blom-Cooper, and Blake Reference Drewry, Blom-Cooper and Blake2007; Masood and Lineberger Reference Masood and Lineberger2020; Masood and Bowie Reference Masood and Bowie2023). The size of the UK judiciary also limits the pervasiveness of ideology. The twelve Supreme Court justices monitor only 38 appellate court judges, and they review approximately 30 to 40 percent of petitions in a given term. The small ratio of higher court justices to lower court judges coupled with considerable auditing capacity severely limits the lower court’s ability to hand down ideological decisions without facing scrutiny or reversal from their Supreme Court superiors (Caminker Reference Caminker1994; Klein and Hume Reference Klein and Hume2003; Posner Reference Posner2005; Bowie and Songer Reference Bowie and Songer2009).

Because the United Kingdom’s career-based judiciary and robust oversight capabilities significantly minimize the extent to which lower court judges write opinions with the sole intention of pursuing their political preferences (Hanretty Reference Hanretty2020), precedent – not ideology – is central to the decision-making process. Supreme Court justices should be less inclined to question the logic and motivations behind Court of Appeal decisions because those decisions are consistently grounded in the precedents that the justices themselves have created and reaffirmed. Therefore, the institutional norm for the appellate court to rely on Supreme Court stare decisis should result in high levels of opinion language borrowing, as the justices can be confident that Court of Appeal decisions align with what the Supreme Court has already established as legitimate law.

Theoretical expectations

Given that there is little empirical evidence showing that attitudinal preferences are central to the UK’s opinion writing and decision-making processes (Malleson and Moules Reference Malleson and Moules2010; Hanretty Reference Hanretty2020), we premise our theoretical expectations on the notion that language borrowing tendencies within the United Kingdom are driven by unique legal and institutional factors rather than distinctly political preferences. Prior studies on hierarchical interactions suggest that in the absence of ideological motivations, institutional (Masood and Lineberger Reference Masood and Lineberger2020) and casespecific factors (Corley, Collins, and Calvin Reference Corley, Collins and Calvin2011) can impact judicial decision-making and opinion writing behavior. As a result, we expect language borrowing to be influenced by a combination of Supreme Courtlevel attributes and Court of Appeal case characteristics.

We begin with a discussion of the Supreme Court level factors that should impact the propensity for justices to borrow language from lower court opinions. Our initial expectation is that the ideological congruence between the Supreme Court and lower court panels should have little to no effect on whether Supreme Court justices borrow lower court opinion language. The UK Supreme Court typically employs pseudo-random panels of five justices when deciding a case, meaning there is uncertainty regarding which set of justices will serve on a given panel. Therefore, there is no reason to expect, a priori, that a particular Supreme Court panel configuration will borrow more or less based on ideological differences in the aggregate. Panel assignment is conditional on each justice’s area of specialization, as well as the justices’ schedules, professional recusals, and workload considerations (Blom-Cooper, Dickson, and Drewry Reference Blom-Cooper, Dickson and Drewry2009; Hanretty Reference Hanretty2017; Hanretty Reference Hanretty2020). Similarly, the mandatory retirement age of 70 creates low membership stability and ultimately engenders additional ambiguity about the composition of justices who will review appellate court decisions (Hanretty Reference Hanretty2020). Random panels along with significant membership turnover make it difficult for lower court judges to consistently predict the ideological preferences of the reviewing justices, meaning lower court judges will write opinions that largely adhere to precedent in order to avoid Supreme Court reversal (Masood and Lineberger Reference Masood and Lineberger2020). As such, we expect Supreme Court justices to borrow language from lower court opinions regardless of the ideological similarities between appellate and Supreme Court panels.

Although we do not expect ideology to play a meaningful role in the justices’ decisions to incorporate lower court language within their own opinions, they should intuitively be more inclined to borrow language when they are affirming a lower court decision. Bowie and Savchak offer that, “when affirming a judgment below, a justice may look to lower court opinions for shortcuts in useful language that may be helpful for explaining their own legal rationale” (Reference Bowie and Savchak2022, 11). We might expect UK Supreme Court justices to frequently consult and borrow language from lower court opinions when they affirm because upholding a lower court decision reflects a mutually agreeable outcome that likely involves overlapping or highly similar legal justifications between the two levels of the judiciary. However, if the Supreme Court reverses a lower court decision, the justices have to articulate original arguments that explain why the lower decision is problematic and why an alternative rationale is needed to justify the legal outcome. In such an instance, it is highly unlikely for the justices to extensively borrow language from the lower court opinion. Therefore, we expect justices to have a greater propensity to borrow language from opinions when they agree with the lower court outcome.

Finally, the size of the reviewing Supreme Court panel should play a considerable role in the justices’ choices to borrow lower court opinion language. We address two theoretical explanations that explain this expectation. First, larger panels reflect comparatively more important decisions (Hanretty Reference Hanretty2020). Although the justices typically make decisions in pseudo-random panels of five, the Court employs larger panels of seven, nine, or even eleven justices when a case is particularly consequential.Footnote ⁹ The UK Supreme Court notes within its rules that cases assigned more than five justices indicate, among other things, “a case of great public importance.” We expect that justices will devote more time to crafting original opinions in salient cases given their gravity and heightened likelihood of public attention (Corley Reference Corley2008; Corley, Collins, and Calvin Reference Corley, Collins and Calvin2011; Hanretty Reference Hanretty2020). Second, compared to the standard five-justice panel, larger panels provide the justices with comparatively more opportunities to deliberate amongst each other, which should reduce the need to borrow from lower court opinions. As such, we expect borrowing rates to be higher when the Supreme Court panel consists of the standard, five-justice configuration as opposed to panels greater than five.

In addition to the Supreme Court attributes discussed above, casespecific characteristics at the appellate level – outside of the control of the justices – may also impact language borrowing tendencies within the UK. First, Supreme Court justices may be more inclined to borrow lower court language when a lower court panel decides a case unanimously. Research demonstrates that judicial decision-makers sitting on courts of last resort (Corely, Steigerwalt, and Ward Reference Corley, Steigerwalt and Ward2013) and lower courts (Hansford and Spriggs Reference Hansford and Spriggs2006; Masood, Kassow, and Songer Reference Masood, Kassow and Songer2019; Masood and Kassow Reference Masood and Kassow2023) are more likely to adopt legal rationales in the absence of a counter signal by a dissenting judge (Beim, Hirsch, and Kastellec Reference Beim, Hirsch and Kastellec2014). Dissenting opinions also provide judicial decision-makers with additional avenues from which to borrow language that might be more persuasive than the content of the lower court majority opinion. The absence of dissenting opinions removes such opportunities that may dissuade justices from borrowing language from the majority opinion.

Previous research also finds that the type of opinion influences whether a higher court borrows from the resulting opinion (Corley, Collins, and Calvin Reference Corley, Collins and Calvin2011). For instance, split decisions may signal to the justices that there is ideological divisiveness or disagreement over the legal justifications on which the decision is based (Masood and Lineberger Reference Masood and Lineberger2020). Such disagreements should decrease the likelihood for the Supreme Court to borrow lower court language, particularly due to the importance that the justices attach to precedent rather than ideological considerations. Because unanimous decisions in the lower court provide justices with a single avenue from which to adopt language and signal that the decision is grounded in legal rationale based on consensus, we expect to see higher rates of borrowing in instances where lower court decisions are unanimous.

Finally, an additional Court of Appeal case characteristic that should impact Supreme Court language borrowing is the length of the lower court opinion. Lengthier opinions, by definition, provide more content for the justices to potentially incorporate within their own opinions. Opinion length might also be indicative of a more thoughtful and deliberative process of crafting an opinion (Leonard and Ross Reference Leonard and Ross2016; Hinkle Reference Hinkle2017). As such, the justices might be more willing to adopt language from lengthier opinions because longer opinions may, in the aggregate, better justify the logic of a lower court decision compared to shorter, less precise lower court justifications. This could be due to the fact that lengthier opinions may be better embedded within existing doctrine and are therefore more persuasive or informational. Lengthier opinions are also arguably applicable to a wider set of factual situations if we assume that longer opinions discuss the applicability of the legal reasoning more thoroughly. For instance, Masood and Lineberger (Reference Masood and Lineberger2020) demonstrate that lengthier opinions supply more information to judicial actors in the UK compared to shorter opinions. Beyond this finding, the justices might view longer opinions as more legitimate because length could imply greater attentiveness and care in articulating the decision. For these reasons, we expect Supreme Court justices to borrow language more extensively from lengthier appellate decisions compared to opinions that are comparatively shorter.

Data and methods

In order to empirically assess how the United Kingdom’s career-based judiciary and institutional norms affect the degree to which justices borrow language from Court of Appeal opinions, we collect data from multiple sources to construct several new and original datasets. First, we collect data from the British and Irish Legal Institute (BAILII) websiteFootnote ¹⁰ to identify the universe of cases decided by the UK Supreme Court between the years 2009 and 2019.Footnote ¹¹ The final dataset includes 548 judicial decisions, where our unit of analysis is the Supreme Court majority opinion – Court of Appeal opinion dyad.Footnote ¹²

To create our dependent variable, we utilize BAILII’s case history functionFootnote ¹³ to access higher and lower court opinion texts. After locating each Supreme Court opinionFootnote ¹⁴ and the accompanying appellate court decision under review, we convert each opinion into a text format and use WCopyfind 4.1.5Footnote ¹⁵ – software which identifies language similarities – to assess the degree to which the lower court opinion words and phrasing were replicated, or “adopted,” in the UK Supreme Court opinion (Corley Reference Corley2008; Corley, Collins, and Calvin Reference Corley, Collins and Calvin2011; Bloomfield Reference Bloomfield2016; Savchak and Bowie Reference Savchak and Bowie2016; Bowie and Savchak Reference Bowie, Savchak, Sterett and Walker2019; Bowie and Savchak Reference Bowie and Savchak2022).Footnote ¹⁶ Our dependent variable is the percentage of a Supreme Court opinion’s language that is borrowed directly from the lower court decision. We set the shortest phrase match to ten words, which tells the software to ignore language matches of nine words or less. This provides more conservative estimates than the frequently used six-word match.Footnote ¹⁷ The dependent variable ranges from 0 to 42 percent.

Table 1 provides an example of the type of borrowing reported by the WCopyfind program. Returning to the case of R v. Gnango, the Supreme Court’s lead opinion highlights that the account of the case facts comes “almost verbatim [from the] judgment of the Court of Appeal.”Footnote ¹⁸ This direct incorporation of the lower court’s language into the Supreme Court opinion does not, however, end with a summary of the facts. Table 1 illustrates one of the many instances of word-for-word language adoption interspersed throughout the Supreme Court’s majority opinion. As the Gnango example demonstrates, the language used in opinions is essential to the judicial process for identifying the issue, clarifying legal doctrine, and establishing guidelines for judges to follow when adjudicating similar cases (Corley, Collins, and Calvin Reference Corley, Collins and Calvin2011).

Table 1. Language Borrowing in R v. Gnango

We include explanatory variables to account for the Supreme Court level attributes and Court of Appeal case characteristics detailed in the theory. Three independent variables capture effects of Supreme Court level attributes on language borrowing. Although we believe that the judiciary in the United Kingdom is predisposed to “getting the law right,” nevertheless, it is necessary to assess whether ideological preferences at the Supreme Court and Court of Appeal level exert any influence on decision-making in the UK Supreme Court. Ideological Congruence measures the ideological similarity between the Supreme Court’s opinion author and the opinion author of the Court of Appeal. Each judge’s ideology is determined by the party of the appointing prime minister.Footnote ²¹ A value of 1 indicates ideological congruence between the Supreme Court’s lead opinion author and the lead author of the Court of Appeal opinion, and a value of 0 indicates otherwise.Footnote ²²

To assess whether UK Supreme Court justices have a preference toward borrowing lower court opinions when they are affirming a decision, we include the Affirm variable that is coded as 1 when the lower court decision is affirmed, and 0 otherwise. We expect a positive estimate for this variable, demonstrating that the justices are more likely to borrow when they are in agreement with the lower court about the application of the law. In addition, we include a variable to capture whether the size of the Supreme Court panel influences whether the justices adopt language from the Court of Appeal opinion. To be consistent with existing measures, Panel Size equals 1 if the case is decided by the standard panel of five justices and 0 if the panel includes more than five justices (Hanretty Reference Hanretty2020; Masood and Lineberger Reference Masood and Lineberger2020).Footnote ²³ The panel size variable should be signed in the positive direction, as justices should be more apt to borrow language in cases that are considered more routine and less salient to the public.

We also include two variables to capture appellate level case characteristics. Unanimous equals 1 if the decision from the Court of Appeal is unanimous, and 0 otherwise. We expect this variable to be positively signed, suggesting that justices tend to borrow more language from the lower court opinion when they are certain there is decisional and jurisprudential agreement among the lower court judges. Finally, we include an Opinion Length variable in order to account for the length of each lower court decision. We first determine the number of words within each appellate opinion and then transform the variable by calculating the natural logarithm of the total number of words. This approach allows us to account for any nonlinearity or variation within the values of the variable (see Randazzo, Waterman, and Fine Reference Randazzo, Waterman and Fine2006; Bowie and Savchak Reference Bowie and Savchak2022). We expect the estimate of this variable to be signed in a positive direction, as it would indicate that justices borrow more language from lengthier opinions compared to those that are less verbose.

Beyond the key variables of interest, we account for several confounders. At the Supreme Court level, we include a control for whether a Supreme Court justice is new to the Court. Freshman Justice equals 1 if the UK Supreme Court opinion writer is within the first two years of serving on the Supreme Court, and 0 otherwise.Footnote ²⁴ We expect a positive estimate, which would account for an initial period of acclimation to the work of the higher court (Hagle Reference Hagle1993; Bowen Reference Bowen1995). Next, we include a control for the Supreme Court’s caseload because a disproportionately higher number of decisions may reduce the amount of time a justice can dedicate to crafting an opinion and increase his or her likelihood to borrow language from the lower court. The Caseload variable is coded based on the number of cases adjudicated by the Supreme Court in each term. Given nonlinearity in the values of this variable, we take the natural logarithm of the total number of cases by term.

Finally, we control for two key factors at the Court of Appeal level that may shape how much (or little) a justice borrows from the Court of Appeal decision. Prior scholarship demonstrates that writing clarity is an important factor affecting the judicial process, as higher levels of opinion clarity increase the transparency and perceived legitimacy of the ruling at hand (Goelzhauser and Cann Reference Goelzhauser and Cann2014; Black et al. Reference Black, Owens, Wedeking and Wohlfarth2016; Nelson and Hinkle Reference Nelson and Hinkle2018; Bowie and Savchak Reference Bowie, Savchak, Sterett and Walker2019; Bowie and Savchak Reference Bowie and Savchak2022). As such, lower court opinions that are clearly written may be more strongly perceived as rationally grounded decisions, meaning we should see higher rates of borrowing from those opinions. To account for these effects, we gauge the clarity of an opinion by taking into account its reading ease as well as the extent to which the opinion uses the passive voice. It stands to reason that it should be easier to decipher and understand the logic of an opinion that is written absent difficult language. Similarly, the absence of passive voice language in legal writing maximizes opinion clarity and decreases the ambiguity of judicial decisions (Wydick Reference Wydick1998; Bowie and Savchak Reference Bowie and Savchak2022). We gather data for two variables meant to capture lower court opinion clarity. The first is Reading Ease and the second is Passive Voice. We collect both pieces of information by copying the lower court opinion text into a Microsoft Word document and running the text through the software’s automated readability tests. Word calculates reading ease using the Flesch-Kincaid readability test, which assigns the document a score between 0 and 100. Higher scores for Reading Ease indicate easier material to read, whereas lower scores indicate more difficulty. The Passive Voice variable reports the percentage of sentences in each lower court opinion that use the passive voice.

Empirical results

Do UK Supreme Court justices frequently borrow language from the opinions of lower court judges? The short answer is yes. We find that justices on the UK Supreme Court directly incorporate some language from the opinions of Court of Appeal judges in over 90% of cases. This suggests an overwhelming embrace of language adoption by the Supreme Court.Footnote ²⁵ We also discover that UK Supreme Court justices adopt, on average, approximately 9% of the language directly from lower court opinions. To better understand how frequently the justices borrow language from lower court opinions, Figure 1 presents the percentage of Supreme Court majority opinions that include borrowed language by justice. The data show that 11 out of 26 justices in our data (42%) borrow language in every opinion for which they are the lead opinion writer. The remaining 15 justices incorporate borrowed language in at least 80% of the majority opinions they author.Footnote ²⁶ The data on language borrowing tendencies are clear: it is the exception – rather than the rule – for a justice to fail to adopt language from lower court opinions.

Figure 1. Percentage of UK Supreme Court Majority Opinions with Borrowed Language, by Author, 2009–2019.

To more systematically explore the factors that influence language borrowing tendencies in the UK Supreme Court, we estimate linear regression models. The estimates presented in Table 2 include three specifications. Model 1 includes justice fixed effects to account for within-group variation among the justices. Model 2 includes justice and year fixed effects to account for both within-group variation among justices and unobserved heterogeneity among Supreme Court terms. Model 3 includes justice and year fixed effects with block boot-strapped standard errors to preserve the original clustering structure while simultaneously accounting for the small number of group level units (Cameron, Gelbach, and Miller Reference Cameron, Gelbach and Miller2008). The results are robust across all model specifications.

Table 2. Regression Models of Language Borrowing at the UK Supreme Court, 2009–2019

Note: The unit of analysis is the UK Supreme Court–Court of Appeal opinion dyad. The dependent variable is the percentage of the lower court opinion that is borrowed by the UK Supreme Court opinion with the shortest phrase match set at ten words. Model 1 includes Justice fixed effects. Model 2 includes Justice and Year fixed effects. Model 3 includes Justice and Year fixed effects with bootstrap standard errors from 1000 block-bootstrap replications given the relatively small number of clustering units (Cameron, Gelbach, and Miller Reference Cameron, Gelbach and Miller2008). *p < .05 (one-tailed tests).

Given the UK judiciary’s unique institutional structure that emphasizes career-based elevation and an apolitical appointment process, we hypothesized that ideological compatibility between opinion authors at the Supreme Court and lower court level should have little to no effect on opinion-borrowing tendencies, all else being equal. The coefficient estimate for Ideological Congruence is not statistically significant. Consistent with our expectation, we find that ideological congruence between higher and lower court opinion authors exerts no meaningful effect in either increasing (or decreasing) the extent to which Supreme Court justices adopt language from lower court opinions. In other words, we find no evidence to indicate that justices on the UK Supreme Court are borrowing language from lower court judges with whom they are ideologically aligned. The absence of ideological influences on language borrowing tendencies corroborates our intuition that a nonpolitical, career-based judiciary seems to minimize the pervasiveness of ideological considerations in both the decision-making and opinion writing processes. The absence of ideological motivations suggests that judges and justices in the UK may prioritize legal or case-specific considerations over ideology, even at the highest tier of the judicial hierarchy.

Next, although we hypothesized that the justices should borrow more language from the lower court opinion when they are affirming that decision, we do not find evidence to support this expectation. The positive yet statistically insignificant coefficient estimate for Affirm suggests that Supreme Court justices borrow language from the appellate court regardless of whether the Court agrees with the lower court’s outcome. This unexpected finding indicates that mere agreement with the legal outcome at the lower court level does not effectuate higher levels of opinion content adoption by UK Supreme Court justices. Instead, the result suggests that justices are just as likely to borrow language from lower court opinions whether they uphold or overturn the lower court decision.

Our final Supreme Court level attribute gauges the impact of a standard versus larger Supreme Court panel. The coefficient for Panel Size is statistically significant and signed in the positive direction. Recall that this variable is coded as 1 when the Supreme Court employs a standard panel configuration of five justices. The results indicate that opinion authors borrow more language from lower court opinions when the Supreme Court panel size is composed of five justices rather than larger panels of seven, nine, or eleven. The standard five-justice panel produces opinions where, on average, approximately 9.1% of the language is borrowed from the lower court opinion. Panels of more than five justices, however, produce opinions where approximately 6.8% of the language comes from the lower court opinion. Because larger panel sizes indicate that a case is of great public importance (i.e., salient), this finding is consistent with research on the American courts showing that Supreme Court justices tend to rely less on lower court opinions in cases that address salient issues (Corley, Collins, and Calvin Reference Corley, Collins and Calvin2011).

Turning to the Court of Appeal level attributes, we do not find any evidence that UK Supreme Court justices borrow more language from unanimous lower court opinions compared to opinions that are nonunanimous. The variable Unanimous has a positive coefficient, yet it is statistically indistinguishable from 0, meaning that agreement among the full panel of lower court judges regarding the outcome of a case does not motivate the justices to borrow more language from the lower court opinion compared to when the lower court decision includes a dissenting opinion.Footnote ²⁷ This result suggests that neither lower court consensus nor the availability of alternative rationales by dissenting judges has any discernible effect on language borrowing tendencies among Supreme Court justices in the UK.

We do, however, find evidence supporting our expectation that the length of a lower court opinion should affect Supreme Court language borrowing patterns. The estimate for Opinion Length is statistically significant and positively signed. Consistent with research showing that US judges borrow more language from longer lower court opinions, so too do UK justices (Corley, Collins, and Calvin Reference Corley, Collins and Calvin2011; Bowie and Savchak Reference Bowie and Savchak2022). Figure 2 illustrates the substantive effect of lower court opinion length on Supreme Court language borrowing.Footnote ²⁸ The results demonstrate that going from the minimum to the maximum value of the Opinion Length variable results in roughly a ten-percentage point increase in language borrowing, which represents a large effect. This result suggests that, ceteris paribus, lower court opinions codified in lengthy expositions and justifications of the outcome increase the tendency among Supreme Court justices to adopt the content of lower court opinions.

Figure 2. Impact of Opinion Length on Language Borrowing in the UK Supreme Court.

Note: The effects are based on the estimates from Model 3. To plot these effects, we generate the predicted probability across all real values in the data. The solid line represents the predicted opinion percentage borrowed by the UK Supreme Court from the UK Court of Appeal. The shaded area represents the 95% confidence intervals.

Finally, turning to our control variables, neither Supreme Court level controls – Freshman Justice and Caseload – reach conventional levels of statistical significance. These results indicate that justices who are newer to the Supreme Court do not experience acclimation effects such that they incorporate lower court language more extensively within their majority opinions compared to their more experienced peers.Footnote ²⁹ Relatedly, the number of cases that the Supreme Court reviews in a given term does not affect language borrowing patterns. This suggests that inevitable variations in the number of cases adjudicated by the Supreme Court, from one term to another, is not consequential in impacting the tendency of the justices to borrow language from lower court opinions. The two Court of Appeal-level controls are also statistically indistinguishable from 0 across all model specifications. Neither Reading Ease nor Passive Voice influence language borrowing patterns, suggesting that neither the textual clarity nor the readability of lower court opinions lead UK Supreme Court justices to borrow more or less language from lower court opinions.Footnote ³⁰