3. Bridging the Gaps: Multi Task Learning for Domain Transfer of Hate Speech Detection

Accurately detecting hate speech using supervised classification is dependent on data that is annotated by humans. Attaining high agreement amongst annotators though is difficult due to the subjective nature of the task, and different cultural, geographic and social backgrounds of the annotators. Furthermore, existing datasets capture only single types of hate speech such as sexism or racism; or single demographics such as people living in the United States, which negatively affects the recall when classifying data that are not captured in the training examples. End users of websites where hate speech may occur are exposed to risk of being exposed to explicit content due to the shortcomings in the training of automatic hate speech detection systems where unseen forms of hate speech or hate speech towards unseen groups are not captured. In this paper, we investigate methods for bridging differences in annotation and data collection of abusive language tweets such as different annotation schemes, labels, or geographic and cultural influences from data sampling. We consider three distinct sets of annotations, namely the annotations provided by Talat (2016), Talat and Hovy (2016), and Davidson et al. (2017). Specifically, we train a machine learning model using a multi-task learning (MTL) framework, where typically some auxiliary task is learned alongside a main task in order to gain better performance on the latter. Our approach distinguishes itself from most previous work in that we aim to train a model that is robust across data originating from different distributions and labeled under differing annotation guidelines, and that we understand these different datasets as different learning objectives in the way that classical work in multi-task learning does with different tasks. Here, we experiment with using fine-grained tags for annotation. Aided by the predictions in our models as well as the baseline models, we seek to show that it is possible to utilize distinct domains for classification as well as showing how cultural contexts influence classifier performance as the datasets we use are collected either exclusively from the U.S. Davidson et al. (2017) or collected globally with no geographic restriction (Talat 2016; Talat and Hovy 2016). Our choice for a multi-task learning set-up is motivated by a number of factors. Most importantly, MTL allows us to share knowledge between two or more objectives, such that we can leverage information encoded in one dataset to better fit another. As shown by Bingel and Søgaard (2017) and Martínez Alonso and Plank (2017), this is particularly promising when the auxiliary task has a more coarse-grained set of labels in comparison to the main task. Another benefit of MTL is that it lets us learn lower-level representations from greater amounts of data when compared to a single-task setup. This, in connection with MTL being known to work as a regularizer, is not only promising when it comes to fitting the training data, but also helps to prevent overfitting, especially when we have to deal with small datasets.