In this paper, we study the problem of identifying the principals and accessories from the fact description with multiple defendants in a criminal case. We treat the fact descriptions as narrative texts and the defendants as roles over the narrative story. We propose to model the defendants with behavioral semantic information and statistical characteristics , then learning the importances of defendants within a learning-to-rank framework. Experimental results on a real-world dataset demonstrate the behavior analysis can effectively model the defendants' impacts in a complex case. In recent years, much previous work has focused on the building of legal assistant systems with different functions, e.g. searching relevant cases for a given query (Chen et al., 2013), predicting charge labels based on the fact description in a criminal case (Luo et al., 2017; Hu et al., 2018; Zhong et al., 2018), generating the interpretable court views from the fact descriptions (Ye et al., 2018). Though having achieved promising results in this field, most of the work only studies the simple cases with only one defendant. However, there exist lots of complex criminal cases in practice, which will involve multiple criminals. In this work, we propose to study the identification of principals and accessories from the fact description in a criminal case . The principal refers to a criminal who organizes and leads criminal groups to carry out criminal activities or plays a major role in joint crimes. Correspondingly, we refer the accessory as the one who plays a secondary or auxiliary role. As the illustration of our task in Fig. 1, given the fact description as well as a list  indicates equal contribution. of defendants, we expect to identify the principals and accessories from the defendants. Since the fact descriptions in criminal cases are usually narrative texts which mostly record the criminal events, we treat the defendants in the fact descriptions as the narrative roles and the protagonists who have the greater impact will be identified as the principles. Narrative comprehension has been studied in NLP for a long time. The traditional method to measure the importances of roles is based on the roles' dispersion over the story (Karsdorp et al., 2012). It supposes that comparing to less important roles, the roles with bigger impact are expected to appear at more places and are more evenly distributed over the story. However, this assumption ignores actions of roles (denoted as behavioral semantic information ), which may be a key factor that estimates their impacts in legal-context scenarios. In this paper, we propose to model a defendant from two perspectives of behavioral semantic information and statistical characteristics . After that, we further learned to estimate their importances with a learning-to-rank framework (Joachims et al., 2007). Our contributions in this paper can be summarized as:  We are the first to identify principals and accessories from complex cases with multiple defendants based on the comprehension of a narrative fact description. We treat the fact descriptions as narrative texts and the defendants as roles in a narrative story. We propose to model a defendant with semantic information and statistical characteristics and estimate his importance within a learning-to-rank framework. Our work is a task related to narrative comprehension. There has recently been a upsurge in re-Fact Description: [   10  ]#[Xie Junbo started to rob. With the consent of Qiu, they followed Zhang to the bridgehead of Zhatou Village in Daxi Tawn. Xie Junbo pushed Zhang to the ground and then stole a bag, which contained a Nokia mobile phone, more than 10 yuan in cash, and a ID card, a bank card and so on.] Defendants: [  ,  ]#[Xie Junbo, Qiu] Principal: [  ] # [Xie Junbo] Accessory: [  ] # [Qiu] Figure 1: An example of a case involving two defendants. search in information extraction of narrative and story understanding. Ouyang and McKeown (2015) presents a change-based model to capture the rise and fall of story characteristics within narrative. Goh et al. (2012) proposes to identity the protagonist in fairy tales automatically with the aid of verbs. Karsdorp et al. (2012) presents a method for extraction the cast from fictional texts and ranks the different cast members on a scale of importance to the story on the basis of their dispersion in the text. However, it only considers the position information and ignores the behavioral semantic information. Meanwhile, the task is related to the researches on legal assistant system. Studies on the application of machine learning in the judicial field have been concentrated in the following directions: learning to predict charges for criminal cases given the fact descriptions (Luo et al., 2017; Jiang et al., 2018; Chao et al., 2019), identifying applicable articles for a given case (Liu and Liao, 2005), providing a tool for automated text summary of legal documents based on word frequency augmented with additional domain-specific knowledge (Polsley et al., 2016). In addition, Ye et al. (2018) put forward a new task of COURT-VIEW-GEN that generates court view from the fact description. But those studies do not involve complex cases involving multiple criminals. Given the fact description f of a case and its defendants set d ( d 1 , d 2 , ..., d n ) , we expect to classify each d i as either a principal or an accessory. A function F for scoring each defendant is learned by a ranking method and we regard its result as the probability of d i being a principal. Note that this could be treated as a classification problem without loss of generality. We consider two feature families when modeling defendant: behavioral semantic features (denoted as f semantic , including Activity Fragments ) and statistical characteristics (denoted as f statistical including Sentence Syntactic Complexity , Cooperation Mode and Order and Frequency ). Activity Fragments : Sentences containing one's actions can reflect his impact in the case to a great extent. Then, we select sentences by name for each defendant and filter out those without verbs. We feed them and the total fact description into two bidirectional LSTMs (Schuster and Paliwal, 1997) for automatic semantic information extraction. Next, we introduce match-lstm (Wang and Jiang, 2016) to fuse those two outputs to measure a defendant's impact on the case. The output from total fact description corresponds to the hypothesis of match-lstm and that from activity fragments corresponds to the premise. Finally, the output of the match-lstm is treated as the behavioral semantic features ( f semantic ) of a defendant. Sentence Syntactic Complexity : The principal of a case is defined as a person who plays a major role in criminal activities. Then, he may appear in more sentences and there may be more verbs related to him. Accordingly, we utilize the syntactic complexity of sentences (Ouyang and McKeown, 2015) as an important feature. Several statistical characteristics are considered to model the syntactic complexity, including the length of the sentence ( sentlength ), the length of its verb phrases ( vplength ), the depth of the sentence's parse tree ( sentdepth ) (Klein and Manning, 2003), the depth of the verb phrase's parse tree ( vpdepth ), average number of words ( avgwords ), average number of verds ( avgverbs ). Cooperation Mode : Moreover, the protagonist is often the plotter of the story and it can be expressed as who is the planner of the case. This information is often reflected by some verbs or con After contacting in advance, Shen Chunxi accompanied Yang Huijun to the Yu's lodge in Fangjia village, Chengbei street 2013  9  11   During the period from September to November 2017, the defendants Tao, Wu and Gao were premeditated Figure 2: Two example that reflects the defendants' cooperation mode in a case. The word in red represents a master-slave relation between two defendants and the word in green represents equality relation. Therefore, we could consider only defendant pairs ( A , B ) such that A plays a more important role than  http:/wenshu.court.gov.cn B and label it 1 . Accordingly, samples containing only principals or accessories are labelled 0 . We get a total of 15312 criminal cases with more than two defendants and the percentage of cases involving only one principal is 67%. Finally, 41342 paired samples are generated. Summary statistics of the data are listed in Tab. To verify the reliability and stability of the model, we perform 10-fold cross-validation in our dataset. junctions and can be obtained by mining the cooperation mode (could be master-slave or equality relation) between defendants as shown in Fig. 2. total cases cases@ 2 cases@ 3 cases@ 4 + 15312 7364 5016 2932 Table 1: Summary statistics of the data. corpus of criminal cases. Then we utilize the Stanford CoreNLP (Manning et al., 2014) to find out the conjunction or verb between two defendants. Finally, it is mapped to a vector based on which set of cooperation mode it belongs to. Order and Frequency : Finally, we propose two other potentially useful features. One is the order of appearances of the defendants and the other is the number of occurrences . We suppose the principal of a case is the plotter and naturally should appears in the fact description earlier. Besides, defendant with more frequent occurrences probably has a greater impact on the case. 3.2 Ranking Model We utilize RankNet (Burges et al., 2005) to train our ranking model. 4.2 Settings The dimension of word embedding is 200 and dimension of hidden states in BiLstm is set to 256. In addition, mini-batch size is set to 32 and the default learning rate of Adam (Kingma and Ba, 2014) is 1 e  3 . 4.3 Baselines Previous studies on the importance distinction of roles in narrative texts are mainly based on statistical features and we are merely exploring solutions to this new problem proposed by this paper. We calculate scores for both f semantic and f statistical respectively and regard their weighted sum as the final probability of being a principal. The scoring units are all composed of linear functions. 4 Experiments 4.1 Data Preparation Extensive experiments are conducted on a real world dataset obtained from Chinese government website  to evaluate our method. Our baselines are as follows:  Frequency : A basic method in which retrieved items are ranked according to their number of occurrences. Dispersion : The basic idea is that more important roles are expected to appear at more places in the story and are more eventually distributed over the story than less important roles (Karsdorp et al., 2012). Frequency & Dispersion : We combine the two methods above as our third baseline. al. (2018), we regard the paragraph start with our court identified that and end with the above facts as the fact description. Burges et al. (2005) shows that training on ties makes little difference. Model P macro R macro F macro Frequency 66.54 63.73 65.10 Dispersion 71.28 69.44 70.35 Frequency & Dispersion 74.15 72.34 73.23 Ours 80.36 79.18 79.77 Table 2: The performances of different role modeling methods. 1. Tab. 2 presents the performances of different role modeling methods. It can be seen that our model achieves a considerable improvement in P macro , R macro and F macro . As shown in Fig. 3, the defendant in red is the mastermind of the case and should be judged to be the principal, despite his low appearances. Position or frequency information does not effectively reflect the status of a role in such samples. However, our method captures this information by the cooperation mode feature between Yin and Zhao , with the help of verb  instructed . 2  1000   1000 Yin instructed Zhao to be a drug seller and sold two small packages of methamphetamine to Wang Peng at the price of 1000 yuan. Zhao collected 1000 yuan of poison money. We compare the performances of our two feature families to explore which one contributes more to the task and the result is shown in Tab. 4. We find that the feature family f semantic achieve better performance in all the evaluation metrics. And its results are even better than our baselines. It reveals that defendant's behavioral semantic information is more valuable than those statistical characteristics. We expect to find a feature conjunction that makes the most sense for modeling role's impact in a story. Like Duan et al. (2010), we use an advanced greedy method to find the best feature conjunction. Given all n (it is 10 in this paper) features we extracted, we construct 2 n feature sets and randomly pick 100 of them. Then, we run the greedy selection algorithm based on the feature set (de-noted as Best ) with the best MAP among those 100 feature sets. Features excluded those in Best are denoted as Ex best and all the extracted features are denoted as Full . We evaluate the Best and each feature in Ex best and if the result is better than the previous one, this feature will be added into the Best . We repeat the process until the Best is no longer updated. Finally, we get the best feature conjunction composed by f semantic , vpdepth , order of appearances , number of occurrences , cooperation mode . To reflect the gap between the Best and the Full , we evaluate their performances on datasets with different numbers of defendants. Tab. 4 illustrates the Best feature set also outperforms the Full feature set when dealing with cases with different numbers of defendants. We are interested in which features in particular are highly valued for role modeling. The importance of each feature is evaluated by the decrease of performance when removing this feature measured from the Best . Fig. 4 reveals the importance of each feature for role modeling. We observe that f semantic plays a very important role. The F macro declines seriously (more than 6 percentage points) when we remove it from the feature set. We suppose that semantic features represent the behavioral information of roles and a defendant's behavior is of great concern in determining his criminal responsibility. The match result of a defendant's actions and global description of the case can effectively model his influence in the whole case. In this paper, we study the task of identifying principals and accessories from the fact description in a complex case. We find a set of effective features for role modeling. and evaluate that the behavioral semantic information is most worthy of attention. We hope to address this problem with a completely semantic-based approach in the future. This work was supported by National Key Research and Development Program of China (Grant No. 2017YFB1402400), and National Natural Science Foundation of China (No. 61976221).
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