1. Introduction
Legislators’ sentiment in daily legislative business carries important information for understanding their behavior. Who will agree or disagree with a bill on the table? How willingly or hesitantly do they do so? These questions are why legislative scholars have been interested in measuring legislators’ sentiment (e.g. Young and Soroka, Reference Young and Soroka2012; Rheault et al., Reference Rheault, Beelen, Cochrane and Hirst2016; Proksch et al., Reference Proksch, Lowe, Wäckerle and Soroka2019; Osnabrügge, Hobolt and Rodon, Reference Osnabrügge, Hobolt and Rodon2021), although it is challenging to do so appropriately. Thanks to the recently developed technique of computational text analysis, political scientists have been increasingly utilizing legislative speech to study legislators’ ideological positions (Lauderdale and Herzog, Reference Lauderdale and Herzog2016; Schwarz et al., Reference Schwarz, Traber and Benoit2017; Rheault and Cochrane, Reference Rheault and Cochrane2020) and topics of concern (Quinn et al., Reference Quinn, Monroe, Colaresi and Radev2010). Sentiment, the research object of our study, is another major area of application; scholars have measured sentiment in the European Parliament (Proksch et al., Reference Proksch, Lowe, Wäckerle and Soroka2019) and the legislatures in Canada (Rheault et al., Reference Rheault, Beelen, Cochrane and Hirst2016), Ireland (Herzog and Benoit, Reference Herzog and Benoit2015), Japan (Takikawa and Sakamoto, Reference Takikawa and Sakamoto2020), the Netherlands (Grijzenhout et al., Reference Grijzenhout, Marx, Jijkoun, Kaal, Maks and van Elfrinkhof2014), the United Kingdom (Slapin and Kirkland, Reference Slapin and Kirkland2020), and the United States (Hopkins and King, Reference Hopkins and King2010).
However, sentiment detection from text is not an easy task. The method of choice is almost always either supervised learning (Herzog and Benoit, Reference Herzog and Benoit2015) or a dictionary-based method (Proksch et al., Reference Proksch, Lowe, Wäckerle and Soroka2019; Slapin and Kirkland, Reference Slapin and Kirkland2020; Takikawa and Sakamoto, Reference Takikawa and Sakamoto2020), and both types require labels generated by human coders for use as training texts or as lexicons of polarity words. Nonetheless, there are three problems. First, human coding is costly. For example, the Lexicoder Sentiment Dictionary (LSD, Young and Soroka, Reference Young and Soroka2012) summarizes three dictionaries in a labor-intensive manner, and each of them is already the result of hard work by humans. Crowdsourcing (Benoit et al., Reference Benoit, Conway, Lauderdale, Laver and Mikhaylov2016; Haselmayer and Jenny, Reference Haselmayer and Jenny2017; Rudkowsky et al., Reference Rudkowsky, Haselmayer, Wastian, Jenny, Emrich and Sedlmair2018) may alleviate the problem to some extent but does not completely solve it. Second, the selection of labels by human coders is subjective, even when human coders are experts (Mikhaylov, Laver and Benoit, Reference Laver, Slava and Benoit2012). This leads to low values of the intercoder agreement that are “common for sentiment analysis tasks” (Grijzenhout et al., Reference Grijzenhout, Marx, Jijkoun, Kaal, Maks and van Elfrinkhof2014, 122). Even with the recently popularized approach of combining word vectors and seed words (Rheault et al., Reference Rheault, Beelen, Cochrane and Hirst2016; Rice and Zorn, Reference Rice and Zorn2021; Watanabe, Reference Watanabe2021; Hargrave and Blumenau, Reference Hargrave and Blumenau2022), it is difficult to avoid arbitrariness in selecting seed words. Third, labeled texts might be domain specific, and their validity is not guaranteed outside the domain where the labels are created (Grimmer and Stewart, Reference Grimmer and Stewart2013; Osnabrügge, Hobolt and Rodon, Reference Osnabrügge, Hobolt and Rodon2021; Rice and Zorn, Reference Rice and Zorn2021). Importing general-purpose dictionaries such as LSD into political science is sometimes unsuccessful.
The core of our approach is that when training models for sentiment detection, we use labels generated by legislators themselves through their own statements. Specifically, we utilize debates in the Japanese national legislature. In the closing debates on a bill just before a vote is taken, legislators sometimes express their opinion toward the bill, making it clear whether they support or oppose the bill. Therefore, these debates represent the ground truth of the speakers’ sentiments. Since government–opposition conflicts are the basic cleavage in the parliamentary system (Herzog and Benoit, Reference Herzog and Benoit2015; Lauderdale and Herzog, Reference Lauderdale and Herzog2016; Rudkowsky et al., Reference Rudkowsky, Haselmayer, Wastian, Jenny, Emrich and Sedlmair2018; Proksch et al., Reference Proksch, Lowe, Wäckerle and Soroka2019; Curini et al., Reference Curini, Hino and Osaki2020), some readers may think that we only have to refer to debate speeches made by government and opposition party leaders. However, government (opposition) party leaders do not necessarily always express positive (negative) sentiments.Footnote 1 Our approach has the same spirit as other machine learning applications where researchers exploit labels generated simultaneously with the input text, such as the product or movie rating (Pang et al., Reference Pang, Lee and Vaithyanathan2002; Fang and Zhan, Reference Fang and Zhan2015; Nguyen et al., Reference Nguyen, Veluchamy, Diop and Iqbal2018). Fernandes et al. Reference Fernandes, Goplerud and Won(2019) exploit bills as prelabeled texts for topics, namely, the jurisdictions of the committees the bills are referred to.
By exploiting the debate texts as our labeled data, we can solve the above three problems. Notably, first, our approach is not costly. In effect, the legislators themselves have already labeled their debates as either positive or negative. Second, the label (i.e., pro or con regarding the corresponding bill) reflects not our subjective judgment but that of the legislators themselves, which is exactly what we aim to study. Third, domain specificity is a strength of our approach, as the domain of the labeled texts is the same as that of the unlabeled texts: legislative speeches. We contribute to the literature on the sentiment analysis of political texts by proposing a new way to construct labeled data by making the most of the constraining nature of legislative institutions (Proksch and Slapin, Reference Proksch and Slapin2015).
Once labeled data are ready, we can train any supervised learning model by using the debate corpus as the training dataset and scale the remaining legislative speeches accordingly. In this article, we utilize bidirectional encoder representations from transformers (BERT, Devlin et al., Reference Devlin, Chang, Lee and Toutanova2018) to train a model for classification and determine the sentiment score of each speech.Footnote 2
We present two substantive analyses that employ our sentiment scores. The first uses our sentiment scores as a dependent variable. We demonstrate that opposition members’ sentiments become negative when they are senior or the next election is approaching. The second task utilizes our sentiment scores as an independent variable. Specifically, we show that the sentiment scores help predict legislators’ undisciplined behavior during the historic vote of no confidence in 1993 and party defection in the following election that led to the breakdown of the decades-long dominance of the Liberal Democratic Party (LDP).
This article proceeds as follows. The next section gives an overview of the structure of the Japanese national legislature and explains our approach. Then, in the following section, we validate our sentiment scores. The penultimate section includes two substantive analyses that use our sentiment scores. Finally, we conclude the article.
2. Method
2.1. Structure of the Japanese legislative process
The Japanese national legislature, the Diet, has a bicameral system composed of the lower and upper chambers. Both the cabinet and groups of lawmakers can submit bills to either chamber, where cabinet bills dominate the legislative agendas. The Speaker of the chamber refers each submitted bill to a committee, which takes a vote after deliberating.
A “debate” in this article is an optional legislative procedure that comes between the end of deliberation and voting in a committee. A committee member debates as a representative of their respective party. In most cases, at most one member from each party (or each group of parties) debates. Members’ debates are a means for expressing a position, usually through set speeches, rather than an exchange of opinions. For 33% of governmental bills, a debate took place in at least one chamber. The probability of a governmental bill being debated increases with its importance or the level of opposition from political parties (Fukumoto, Reference Fukumoto2000, 34).
As an official legislative procedure, such debates commonly have a specific structure. At the beginning of the debate, legislators explicitly indicate 1) that they deliver the current speech as a debate, 2) whether they are for or against the agenda, and 3) which party they represent. At the end of the debate, legislators usually wrap up their argument, repeating their position regarding the agenda, and explicitly declare that they are closing their debate.
After a vote is taken in the committee, the bill is reported back to the floor, where the chamber takes a vote for the final decision. Debates rarely take place on the floor. If the floor of the first chamber passes the bill, it is sent to the other chamber, which repeats the same procedure. If the second chamber approves the bill, it is enacted as a law.
2.2. Corpus
The corpus for this research consists of speeches included in the minutes of all standing committee meetings of both chambers during 1955–2021.Footnote 3 Note that most speeches in the legislature take place in committees rather than in the plenary. Our unit of observation is a “speech,” noninterrupted lines of dialog spoken by a speaker. (In this article, the lowercase word “speaker” refers to an individual who delivers a speech and does not indicate the Speaker of a chamber.) We retain only speeches made by rank-and-file committee members and discard speeches made by committee chairs, government officials (ministers and bureaucrats), and witnesses, the latter of whom mostly come from the private sector. Our corpus contains 
$3,148,817$ speeches in total.Footnote 4
2.3. Sentiment score
2.3.1. Extracting labeled speeches
To make a labeled dataset, we first extract debate speeches. We regard a speech as a “debate speech” when the first sentence of the speech satisfies the following three conditions. 1) It contains the word “tōron” (debate) or the single Chinese character “i,” which is often a part of “iken” (opinion) or “ishi” (intention). 2) It contains words indicating the speaker’s position toward the agenda, either “sansei” (agree) or “hantai” (oppose) but not both.Footnote 5 Accordingly, we also label the debate speech’s position as either pro or con. 3) It contains the word “daihyō” (represent), which indicates that the speaker is debating on the behalf of a party or parties. We read dozens of randomly chosen debate speeches defined above and confirmed that all of them are actually debates.
As we explained above, in the opening and closing of a debate speech, the speaker explicitly states his or her position and party. A supervised classifier would rely heavily on the information from stylized language such as “agree” and “disagree” or the party name of the speaker, ignoring the sentiment in the speech. That being the case, the resultant model would not be able to classify unlabeled speeches that lack those words. To prevent such a problem, we split every debate speech into three tertiles with a roughly equal number of sentences and use only the second tertile to train a supervised learning model.Footnote 6 For the same reason, we remove words with the characters “san” or “han”, which can be part of “sansei” (agree) or “hantai” (oppose) in both debate and nondebate speeches.Footnote 7
Ultimately, we find 1,928 pro-side debate speeches and 5,428 con-side debate speeches.Footnote 8 These are the “labeled speeches” in this article. We randomly sample 60% (
$N=4,413$) of the labeled speeches as the training dataset, 20% (
$N=1,471$) as the development dataset, and the remaining 20% (
$N=1,472$) as the test dataset.
2.3.2. Training a supervised learning model
Once we have labeled the speeches, we can train any supervised learning model. That said, in this article, we use a language representation model, BERT (Devlin et al., Reference Devlin, Chang, Lee and Toutanova2018). One merit of BERT models is their ability to capture the context where every word is situated using its bidirectional transformer mechanism, which surpasses traditional bag-of-words approaches. Specifically, we employ a pretrained BERT model for the Japanese language, which can be fine-tuned for many downstream natural language processing tasks. Our downstream task is to classify speeches into pro- and con-side speeches.Footnote 9
We fine-tune the model by using the second tertile of each speech in the training dataset so that it can predict the label of each speech. Then, we evaluate the model’s performance in terms of the loss function of cross-entropy by using the second tertile of each speech in the development dataset. We originally planned to repeat this procedure for several epochs until the model performance stopped improving for three epochs in a row so that we could avoid overfitting. In fact, as the model metric did not improve after the first epoch, we use the model from the first epoch.
2.3.3. Calculating the sentiment scores of speeches
Once the fine-tuning of the BERT model is completed, we can process every speech through the model, regardless of whether the speech is labeled. In the last stage of processing, the fine-tuned BERT model produces the probability that the speech is a pro-side speech. We multiply the probability by 100 and use it as our sentiment score for the speech. If the sentiment score is closer to 100 (or 0), the speech is considered to be more positive (negative).
3. Validation
For any text analysis, validation is a necessary step to check if the method works as expected (Grimmer and Stewart, Reference Grimmer and Stewart2013). In this section, we carry out a series of validity checks using the in- and out-sample labeled speeches, the unlabeled speeches, and human-based and dictionary-based sentiment scores.
3.1. Labeled speeches
Using labeled speeches, we evaluate the performance of the BERT model. As explained in the previous section, we divide each labeled speech into three tertiles, and for fine-tuning, we use only the second tertile of each labeled speech in the training and development datasets, which consist of in-sample labeled speeches. We explore the classification performance for out-sample labeled speeches, namely, the second tertiles of labeled speeches in the test dataset. The performance metrics are presented in Table 1, where we classify a speech as pro-side (or con-side) speech if the sentiment score is greater (less) than 50. Two F1 metrics, pro-F1 and con-F1, are the values of F1 for the pro and con classes, respectively.Footnote 10 Each row shows the metrics achieved for specific data subsets. For the second tertile of each labeled speech in the training and development datasets (in the first row), unsurprisingly, the accuracy and the two F1 scores are truly high (c.f., Devlin et al., Reference Devlin, Chang, Lee and Toutanova2018). When we turn to the second tertile of each labeled speech in the test dataset (in the second row), the values of the three metrics are still high.
Classification performance of the BERT model using labeled speeches
Note: The unit of observation is a labeled speech. We randomly sample 60% of the labeled speeches as the training dataset; 20% as the development dataset; and the remaining 20% goes to the test dataset. We also divide each labeled speech into three tertiles. In the third row, we use the original nonsplit labeled speeches.
Eventually, we will scale every speech without dividing it. Thus, in the third row of Table 1, we report the three metrics for the nonsplit labeled speeches in the test dataset, which are better than their second tertile counterparts (second row) and even than those of the second tertile of each labeled speech in the training and development datasets (first row). This improved performance is probably due to the increased amount of information from the texts due to not splitting the labeled speeches. In the Supplementary Materials, we demonstrate that the model also classifies the first and third titles successfully, and the performance of the model does not change over time. These results demonstrate that our BERT model works quite well for out-sample labeled speeches.
3.2. Unlabeled speeches
We now use the unlabeled speeches for validation (
$N=3,141,461$). Since most agendas are proposed by the government and most unlabeled speeches are technically questions posed by legislators to the government, scholars expect government members’ sentiments to be more positive than opposition members’ sentiments (Herzog and Benoit, Reference Herzog and Benoit2015; Lauderdale and Herzog, Reference Lauderdale and Herzog2016; Rudkowsky et al., Reference Rudkowsky, Haselmayer, Wastian, Jenny, Emrich and Sedlmair2018; Proksch et al., Reference Proksch, Lowe, Wäckerle and Soroka2019; Curini et al., Reference Curini, Hino and Osaki2020). In our corpus, 11.9% of speeches are from government parties, and 88.1% are from the opposition.Footnote 11 Figure 1 illustrates the densities of sentiment scores of government (solid line) and opposition members (dotted line). As expected, the sentiment scores of government members tend to be higher, while those of opposition members tend to be lower.Footnote 12 This figure also reveals a little-known substantive fact: even government members sometimes (or quite often) make negative speeches.Footnote 13
Sentiment score distribution by government status.
We also expect that when a party switches from the government to the opposition (the opposition to the government), the sentiment scores of its members’ speeches decrease (increase). To visualize this change, we present the time series of the annual average sentiment scores of the two largest parties (Figure 2). For each year and party, we calculate the average sentiment scores of the speeches made in the session in which the original national budget was passed.Footnote 14 As the LDP was in the government for most of the study period (except for 1994 and 2010–2012, which are shaded in the figure), we contrast the LDP (black dots, solid line) with its major opponent (gray dots, dashed line), which is the largest opposition party when the LDP is in the government and the largest party in the government when the LDP is in the opposition.Footnote 15 It is clear that the sentiment score of a party is higher when the party is a government party than when it is an opposition party. This pattern implies that our sentiment score captures not an ideal point but rather a sentiment.
Annual average sentiment scores for the two largest parties.
3.3. Human-coding
Another way to check whether our sentiment score is a valid measure of legislators’ sentiment is to examine whether it is consistent with human judgment. To obtain human-coded data, we created 12 sets of 65 pairs of speeches. Each pair was assessed by 12 graduate students, who determined which speech was more positive. The Bradley and Terry method (Bradley and Terry, Reference Bradley and Terry1952) was applied to estimate the latent sentiment for each speech. Then, by exploiting the human-based sentiment score as a ground truth, we explored whether, and how well, the legislator-supervised sentiment score aligned with the human-based sentiment score.
Below is the process for creating the 65 pairs of speeches.Footnote 16 We randomly chose 10 nondebate speeches for every ten-point interval of rounded sentiment scores between 10 and 90. Since few nondebate speeches have scores below 0.5 or above 99.5, we randomly chose 10 speeches with scores of 0 (or 100) from nondebate speeches with scores between 1 and 2 (or between 98 and 99). We also randomly chose 10 second-tertiles of pro-side debate speeches and 10 second-tertiles of con-side debate speeches. Thus, we obtained 
$10 \times 11 + 10 \times 2 = 130$ speeches. For each human coder, we generated randomly matched 65 pairs of speeches. The order of 65 pairs was also randomized.Footnote 17
We estimated the sentiment score of each of the 130 speeches included in the 65 pairs by applying the Bradley–Terry model with no explanatory variables. This model assumes that there is a latent score attached to each speech and that for each pair of speeches, a coder is more likely to choose the speech with a higher score. We refer to the latent score for each speech assigned by the model as the human-based sentiment score; below, we refer to our sentiment score as the legislator-supervised sentiment score.
Figure 3 shows the relationship between the human-based sentiment score and the legislator-supervised sentiment score. In the left panel, the box plots display the distribution of the human-based sentiment scores for a group of the 10 nondebate speeches corresponding to each tenth rounded value (0, 10, …, 100) of legislator-supervised sentiment scores, arranged from left to right. Overall, the human-based sentiment scores are positively related to the legislator-supervised sentiment scores (correlation coefficient of 0.646).Footnote 18
Box plots of human-based sentiment scores.
The right panel of the figure provides the box plots of the human-based sentiment scores for the 10 pro-side and the 10 con-side debate speeches in the right and left columns, respectively. The human-based sentiment scores of the pro-side debate speeches tend to be positive, while those of the con-side debate speeches tend to be negative.
In the Supplementary Materials, we also demonstrate that the legislator-supervised sentiment score explains the judgements of the human coders very well. Overall, human coding has shown that the legislator-supervised sentiment score successfully represents legislators’ sentiment.
3.4. Dictionary-based sentiment score
As a final validity check, we compare the legislator-supervised sentiment scores with those calculated from existing dictionaries. As we noted before, the performance of dictionaries in text analysis is domain-specific and may underperform in some domains of application. We examine the case of parliamentary speech.
The landscape of sentiment dictionaries in Japanese is very different from that in English, where there are a number of existing reputable dictionaries. There is no standard dictionary of this kind in Japanese. Nevertheless, we employ the dictionary by Takamura et al. Reference Takamura, Inui and Okumura2005 because it is relatively popular.Footnote 19 We calculate the dictionary-based sentiment score for each speech as an average of the scores of words and phrases matched to the dictionary, weighted by the inversed document frequency.Footnote 20
We compare three sentiment scores we so far have obtained: legislator-supervised, human-based, and dictionary-based. To evaluate their relationship, we calculate the correlation coefficients between them. When focusing on the correlation coefficients with the human-based sentiment score, the legislator-supervised sentiment score has a higher value (0.646) than the dictionary-based sentiment score does (0.195). This finding suggests that the legislator-supervised sentiment score extracts the sentiment of parliamentary speeches more successfully than the dictionary-based sentiment score does.
In the Supplementary Materials, we also show that a legislator-supervised sentiment score model outperforms a dictionary-based sentiment score model in terms of classification task performance using human-coded data.
4. Analyses
Having established the validity of our sentiment score in the previous section, we now present two substantive analyses using this score.
4.1. Determinants of sentiment
In this first analysis, we explore how electoral and legislative factors affect legislators’ sentiments.
4.1.1. Electoral system
Until the 1993 election, the lower chamber used the single nontransferable voting (SNTV) system, where the district magnitude M is three to five in most districts, each citizen has just one vote, votes cannot be transferred between candidates, and the seats are simply awarded to the top M vote-getters in each district. After the 1996 election, the chamber adopted a two-tier system. The lower tier consists of single-member districts (SMD, M = 1), while the upper tier is composed of closed-list proportional representation (PR) districts (
$6 \leq M \leq 33$). The upper chamber employs another two-tier system. The lower tier employs the SNTV system (
$1 \leq M \leq 6$). The upper tier consists of the national at-large district that used SNTV (M = 50) until 1980, closed-list PR (M = 50) from 1983 to 1998, and open list PR (
$48 \leq M \leq 50$) after 2001.
4.1.2. Data
The unit of observation is a speech. We use both labeled and unlabeled speeches. The dependent variable, Sentiment Score, is our legislator-supervised sentiment score. Below, we introduce four independent variables, referring to prior research (Herzog and Benoit, Reference Herzog and Benoit2015; Proksch et al., Reference Proksch, Lowe, Wäckerle and Soroka2019).Footnote 21
To begin, for members of the lower chamber, we define the variable Seniority as the number of terms they have served. The term of members of the upper chamber is six years, which is approximately twice as long as the average years (2.8 years) for which members of the lower chamber actually served during the study period given terminations due to early elections.Footnote 22 Thus, for members of the upper chamber, we calculate Seniority by doubling the number of terms they served, and we subtract one from this number when they are still in the first half of their current term (c.f., Sato and Matsuzaki, Reference Sato and Matsuzaki1986, 366). Senior legislators deploy sentiment in their speeches more efficiently and strategically than junior legislators (Osnabrügge, Hobolt and Rodon, Reference Osnabrügge, Hobolt and Rodon2021). Therefore, the coefficient of Seniority is expected to be positive for the government and negative for the opposition.
Second, we define the variable Electoral Time as the number of years that have passed between the day of the speech and the election day when the legislator was most recently (re-)elected. Our prediction is that as the next election approaches, government and opposition members increase the contrast with their competitors by expressing positive and negative sentiments in their speeches, respectively (Proksch et al., Reference Proksch, Lowe, Wäckerle and Soroka2019; Crabtree et al., Reference Crabtree, Golder, Gschwend and Indridason2020; Ishima, Reference Ishima2024, 122). Accordingly, as Electoral Time increases, sentiment scores become higher for the government and lower for the opposition.
Third, we define the variable Legislative Time as the number of months that have passed between the day of the speech and the beginning of the current session. Usually, a few sessions are held every year. We expect that toward the end of each session, government and opposition members compromise on bills and thus tone down the sentiment in their speeches. Therefore, the higher the Legislative Time, the lower and higher the sentiment scores that government and opposition members tend to have, respectively.
Finally, we use the variable Electoral Strength, the number of votes received by a legislator in the previous election divided by the Droop quota of his or her district (Cox and Rosenbluth, Reference Cox and Rosenbluth1995. See also Reed and Scheiner, Reference Reed and Scheiner2003; Desposato and Scheiner, Reference Desposato and Scheiner2008, and Saito, Reference Saito2009).Footnote 23 This is missing for legislators elected from closed-list PR districts. We expect that the sentiment scores of electorally vulnerable government (opposition) members are high (low ) because such legislators try to appeal to the electorate by speaking in a charged manner. Thus, the coefficient of Electoral Strength will be negative for the government and positive for the opposition.
4.1.3. Results
As explained in the previous subsubsection, we expect that the independent variables affect sentiment in a different way depending on the type of legislators. Accordingly, we divide the whole dataset by government status and chamber (and electoral system) and analyze each data subset separately.
Figure 4 presents the coefficient plots.Footnote 24 Each row of panels corresponds to an independent variable. The left and right columns of panels correspond to the cases of government and opposition members, respectively. In each panel, the three black dots indicate the lower chamber, while the three white dots indicate the upper chamber. In the first analysis for each chamber (Analysis 1 for Lower Chamber and Analysis 4 for Upper Chamber), indicated in circles, we regress the sentiment score on Seniority, Electoral Time, and Legislative Time as well as legislator fixed effects but not on Electoral Strength so that we can analyze legislators from all types of electoral systems, including PR legislators. In the second and third analyses (triangle and square) for each chamber, we include Electoral Strength as an additional independent variable. In the case of the lower chamber, Analyses 2 and 3 use legislators elected from SNTV and SMD, respectively. In the case of the upper chamber, Analyses 5 and 6 use legislators elected from the lower tier and the pre-1980 upper tier under the nationwide SNTV, respectively.Footnote 25 The dots show the point estimates of the coefficients with the bars for the 95% confidence intervals calculated from the standard errors clustered by legislators.
Summary of regression results: coefficient estimates.
For opposition members, Seniority has significantly negative effects, as anticipated, while for government members, the effects are insignificant. All coefficients of Electoral Time are negative and significant for opposition members, as expected. However, contrary to our expectations, they are mostly insignificant for government members. In Analyses 1 and 3 of the lower chamber, the effects of Legislative Time are significantly negative for government members and significantly positive for opposition members, which is consistent with our argument. In the upper chamber, the coefficients of Legislative Time remain (mostly significantly) negative for government members, although surprisingly, they turn significantly negative for opposition members. Electoral Strength has insignificant coefficients for government members and (sometimes significantly) negative coefficients for opposition members, which we did not expect. In summary, it is fair to conclude robustly that opposition members tend to express negative sentiments when they are senior or the next election is approaching.
4.2. The rebellion and defection of LDP members in 1993
In the second analysis, we explore whether the sentiment score predicts legislators’ behavior at the historic 1993 vote of no confidence in the Ki’ichi Miyazawa Cabinet and the following split-up of the LDP. In principle, parliamentary democracies rarely experience rebellion (Kam, Reference Kam2009). Japan is an extreme case: during the LDP government periods, the party has experienced as few as four instances of rebellions where some members voted against the party discipline.Footnote 26 Among them, only the vote of no confidence in 1993 resulted in a change of the government parties. This is why we focus on this case. If our sentiment score truly measures the heterogeneity of sentiments even among government members, it should be helpful for predicting who would rebel and/or defect.Footnote 27
4.2.1. Background
In 1993, the LDP was divided over “political reform.”Footnote 28 The main issue was whether it would change the SNTV electoral system of the lower chamber. In the 126th session of the legislature (from January 22 to June 18, 1993), the pro-political reform group of LDP members (called “reformers”) managed to submit political reform bills to the lower chamber. Opposition members also proposed their own political reform bills. The cabinet did not submit any bills on this matter. Both sets of bills were referred to the Special Committee for Research on Political Reform (hereafter, the Reform Committee). There, advocates of the reform argued for the bills, while the anti-reform group of LDP members resisted them.
To the disappointment of the reformers, however, the leadership of the LDP decided not to pass the bills toward the end of the session. Opposition parties submitted a motion of no confidence in the Miyazawa Cabinet. As the LDP had a majority in the lower chamber, it would have defeated such a motion under normal circumstances. This time, however, the motion was passed because a number of the reformers, mostly members of the Hata faction, rebelled and supported the motion. Prime Minister Miyazawa then dissolved the lower chamber. As many expected, 44 members from the Hata faction defected from the LDP and formed a new party, the Japan Renewal Party (“Shinseito”). However, it came as a surprise when an additional 10 lawmakers, most of whom did not rebel in the vote of no confidence, also left the LDP and launched the New Party Harbinger (“Shinto Sakigake,” hereafter the NPH). After the general election, these two new parties and six other ex-opposition parties formed a coalition government that ousted the LDP from power for the first time in almost four decades.
4.2.2. Data
The unit of observation is an LDP member who spoke at the Reform Committee in the 126th session (N = 39). We have two dependent variables. One is a dummy variable of whether the LDP member supported the motion of no confidence in the Miyazawa Cabinet on June 18, 1993 (Rebel). There were 25 nay votes, 8 yes votes, and 6 abstentions, the last of which we regarded as missing values. The other dependent variable is a dummy variable of whether the legislator defected from the LDP in the general election on July 18, 1993 (Defect). Since every member ran for reelection, there are no missing values. Note that these two dependent variables are slightly different; all eight rebels defected, five of six absentees did not defect from the LDP, and four members did not rebel but did defect (to the NPH).
The main independent variable, Sentiment Score, is the average of our legislator-supervised sentiment scores of the LDP member’s speeches at 18 meetings of the Reform Committee (January 22 to May 25, 1993) in the 126th session. Note that the sentiment score represents the sentiment toward the political reform bills, not toward the government, because all of the agendas in the Reform Committee were the political reform bills.Footnote 29 Since the leadership of the LDP killed the bills at the last minute, we expect that the higher Sentiment Score is, the more likely the LDP member is to have rebelled or defected. Note also that there were no debate speeches because no vote over the bills was taken at the Reform Committee.
The control variables are Seniority and Electoral Strength (in the 1990 general election), as defined in the previous subsection, which prior research also refers to as determinants of the same dependent variables (Cox and Rosenbluth, Reference Cox and Rosenbluth1995; Kato, Reference Kato1998; Reed and Scheiner, Reference Reed and Scheiner2003; Desposato and Scheiner, Reference Desposato and Scheiner2008; Saito, Reference Saito2009).
4.2.3. Results
We employ logistic regression models. Table 2 displays estimates of the coefficients and the standard errors in brackets. In the left column, we report the results where the dependent variable is Rebel. The coefficient of Sentiment Score is significantly positive, as expected. That is, the more positive an LDP member’s speech is toward the political reform bills, the more likely the member is to have defied the party in the vote of no confidence. The coefficients for Seniority and Electoral Strength are not significant. In the left panel of Figure 5, the line shows the predicted probabilities of rebellion by Sentiment Score, with Seniority and Electoral Strength at their respective median values. The tick marks at the top and bottom represent Sentiment Scores of rebelling and nonrebelling legislators, respectively. The finding that sentiments are predictive of rebellions is not trivial in light of the previous research. For instance, Slapin and Kirkland (Reference Slapin and Kirkland2020, 165) report that “[t]here is little clear pattern between the presence of negative and positive sentiment and rebellion.”
Predicted probabilities of LDP members’ rebellion and defection in 1993 by Sentiment Score. (a) Rebel and (b) Defect.
Regression of LDP members’ rebellion and defection in 1993
Note: 
$^{*}p \lt $ 0.05. The unit of observation is an LDP member on the Reform Committee in 1993. We regress Rebel or Defect on the three independent variables. We report standard errors in parentheses.
A similar pattern arises in the analysis of defection from the LDP as the dependent variable (right column of Table 2 and right panel of Figure 5). The effect of Sentiment Score is positive, which is consistent with our argument, while it comes near reaching a conventional level of significance (p = 0.079). The effects of Seniority and Electoral Strength are not significant.
Overall, our findings imply that observers should be able to predict who is likely to rebel or defect if they listen to legislators’ speeches carefully.
5. Conclusion
The general message of this article in a broader context is simple: for scholars of text analysis who conduct supervised learning and thus need labeled texts, it is advisable to explore and exploit, if any, training texts that have already been labeled (as either positive or negative in the case of analysis of sentiment polarity) by those who generate the texts (Pang et al., Reference Pang, Lee and Vaithyanathan2002; Fang and Zhan, Reference Fang and Zhan2015; Nguyen et al., Reference Nguyen, Veluchamy, Diop and Iqbal2018; Fernandes et al., Reference Fernandes, Goplerud and Won2019). We showcase a good example of the Japanese legislature, where the highly structured debates generate such prelabeled texts available for scientific data analysis.
In our substantive analyses, we show that the sentiment score is useful both in studying the nature of party politics over time and in explaining legislators’ behavior in a historic case. In the first analysis, it turns out that the seniority of legislators affects sentiment polarization by making senior opposition legislators more negative in their speeches; in addition, as the next election approaches, opposition members express more negative sentiments. In the second analysis, government members who expressed positive sentiment toward the political reform bills tended to vote for the motion of no confidence in the cabinet that killed the bills.
The core idea of our process for extracting sentiments from prelabeled debate speeches is applicable to legislative corpora in countries other than Japan. As Proksch and Slapin (Reference Proksch and Slapin2015, 1) observed, “in all parliaments, members debate bills before they vote on them.” For instance, in the U.S. House of Representatives, after a committee reports a bill to the floor, one hour of general debate on the bill follows on the floor. This time is divided evenly between the majority and minority parties, their lawmakers deliver prepared speeches, and the party leaders are informed of “member sentiment and the mood of the House” toward the bill (Oleszek et al., Reference Oleszek, Oleszek, Rybicki and Heniff2020, 203–205, 211–213, 217, emphasis added). Other examples are corpora consisting of debates that have taken place in the European Parliament (Benoit et al., Reference Benoit, Conway, Lauderdale, Laver and Mikhaylov2016) and the Swiss National Council (Schwarz et al., Reference Schwarz, Traber and Benoit2017). For these corpora, scholars may be able to exploit the associated debates as automatically labeled texts, as we do for the Japanese Diet. Or at least, hopefully, political scientists can look for patterns in speech where, under a small set of assumptions, labels can be reliably extracted.
Some readers may question whether labeled and unlabeled speeches are comparable. Their comparability is the basis for the effectiveness of our classifier when applied to unlabeled speeches. A few of our responses are presented below. First, we detect sentiments that are expressed in the same way as sentiments presented by legislators in closing debates, which are politically salient. The types of sentiments expressed in these speeches are precisely what we (and we believe many scholars) are interested in. Second, the Supplementary Materials show that the saliency of debate speeches does not affect the performance of our classifier. Third, most unlabeled speeches are technically questions, although they are mostly legislators’ arguments made under the name of “questions” akin to labeled speeches (i.e., debate speeches). That said, our classifier may be less effective when applied to unlabeled speeches that are less salient or segments of unlabeled speeches that are purely procedual questions.Footnote 30
In summary, we hope that our article contributes to the literature on sentiment analysis and enhances the understanding of legislators’ behaviors, particularly under parliamentary systems.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/psrm.2025.10048. To obtain replication material for this article, please visit https://doi.org/10.7910/DVN/6K5E3Y.
Acknowledgements
We would like to thank Diego Escobari, Etienne Gagnon, Masataka Harada, Musashi Hinck, Silvia Kim, Da-chi Liao, Go Murakami, Marlene Mußotter, Ludovic Rheault, Jonathan Slapin, and Arthur Spirling for their helpful feedback. We appreciate American Journal Experts for providing their proofreading service. We are entirely responsible for the scientific content of the article, and the article adheres to the journal’s authorship policy.
Funding statement
This work was supported by the Japan Society for the Promotion of Science [grant numbers KAKENHI JP16K13340 and JP19K21683]. The financial sponsor played no role.
Competing interests
The authors declare none.
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