Measuring Bias in Contextualized Word Representations

06/18/2019 ∙ by Keita Kurita, et al. ∙ Carnegie Mellon University 0

Contextual word embeddings such as BERT have achieved state of the art performance in numerous NLP tasks. Since they are optimized to capture the statistical properties of training data, they tend to pick up on and amplify social stereotypes present in the data as well. In this study, we (1) propose a template-based method to quantify bias in BERT; (2) show that this method obtains more consistent results in capturing social biases than the traditional cosine based method; and (3) conduct a case study, evaluating gender bias in a downstream task of Gender Pronoun Resolution. Although our case study focuses on gender bias, the proposed technique is generalizable to unveiling other biases, including in multiclass settings, such as racial and religious biases.

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1 Introduction

Type-level word embedding models, including word2vec and GloVe Mikolov et al. (2013); Pennington et al. (2014), have been shown to exhibit social biases present in human-generated training data Bolukbasi et al. (2016); Caliskan et al. (2017); Garg et al. (2018); Manzini et al. (2019). These embeddings are then used in a plethora of downstream applications, which perpetuate and further amplify stereotypes Zhao et al. (2017); Leino et al. (2019). To reveal and quantify corpus-level biases is word embeddings, Bolukbasi et al. (2016) used the word analogy task Mikolov et al. (2013). For example, they showed that gendered male word embeddings like he, man are associated with higher-status jobs like computer programmer and doctor, whereas gendered words like she or woman are associated with homemaker and nurse.

Contextual word embedding models, such as ELMo and BERT Peters et al. (2018); Devlin et al. (2019) have become increasingly common, replacing traditional type-level embeddings and attaining new state of the art results in the majority of NLP tasks. In these models, every word has a different embedding, depending on the context and the language model state; in these settings, the analogy task used to reveal biases in uncontextualized embeddings is not applicable. Recently, May et al. (2019) showed that traditional cosine-based methods for exposing bias in sentence embeddings fail to produce consistent results for embeddings generated using contextual methods. We find similar inconsistent results with cosine-based methods of exposing bias; this is a motivation to the development of a novel bias test that we propose.

In this work, we propose a new method to quantify bias in BERT embeddings (§2). Since BERT embeddings use a masked language modelling objective, we directly query the model to measure the bias for a particular token. More specifically, we create simple template sentences containing the attribute word for which we want to measure bias (e.g. programmer) and the target for bias (e.g. she for gender). We then mask the attribute and target tokens sequentially, to get a relative measure of bias across target classes (e.g. male and female). Contextualized word embeddings for a given token change based on its context, so such an approach allows us measure the bias for similar categories divergent by the the target attribute (§2

). We compare our approach with the cosine similarity-based approach (§

3) and show that our measure of bias is more consistent with human biases and is sensitive to a wide range of biases in the model using various stimuli presented in Caliskan et al. (2017). Next, we investigate the effect of a specific type of bias in a specific downstream task: gender bias in BERT and its effect on the task of Gendered Pronoun Resolution (GPR) (Webster et al., 2018). We show that the bias in GPR is highly correlated with our measure of bias (§4). Finally, we highlight the potential negative impacts of using BERT in downstream real world applications (§5). The code and data used in this work are publicly available.111https://bit.ly/2EkJwh1

2 Quantifying Bias in BERT

BERT is trained using a masked language modelling objective i.e. to predict masked tokens, denoted as [MASK], in a sentence given the entire context. We use the predictions for these [MASK] tokens to measure the bias encoded in the actual representations.

We directly query the underlying masked language model in BERT222For all experiments we use the uncased version of BERTBASE https://storage.googleapis.com/bert_models/2018_10_18/uncased_L-12_H-768_A-12.zip. to compute the association between certain targets (e.g., gendered words) and attributes (e.g. career-related words). For example, to compute the association between the target male gender and the attribute programmer

, we feed in the masked sentence “[MASK] is a programmer” to BERT, and compute the probability assigned to the sentence ‘

he is a programmer” (). To measure the association, however, we need to measure how much more BERT prefers the male gender association with the attribute programmer, compared to the female gender. We thus re-weight this likelihood using the prior bias of the model towards predicting the male gender. To do this, we mask out the attribute programmer and query BERT with the sentence “[MASK] is a [MASK]”, then compute the probability BERT assigns to the sentence ‘he is a [MASK]” (). Intuitively, represents how likely the word he is in BERT, given the sentence structure and no other evidence. Finally, the difference between the normalized predictions for the words he and she can be used to measure the gender bias in BERT for the programmer attribute.

Generalizing, we use the following procedure to compute the association between a target and an attribute:

  1. Prepare a template sentence
    e.g.“[TARGET] is a [ATTRIBUTE]”

  2. Replace [TARGET] with [MASK] and compute =P([MASK]=[TARGET] sentence)

  3. Replace both [TARGET] and [ATTRIBUTE] with [MASK], and compute prior probability

    =P([MASK]=[TARGET] sentence)

  4. Compute the association as

We refer to this normalized measure of association as the increased log probability score and the difference between the increased log probability scores for two targets (e.g. he/she) as log probability bias score which we use as measure of bias. Although this approach requires one to construct a template sentence, these templates are merely simple sentences containing attribute words of interest, and can be shared across multiple targets and attributes. Further, the flexibility to use such templates can potentially help measure more fine-grained notions of bias in the model.

In the next section, we show that our proposed log probability bias score method is more effective at exposing bias than traditional cosine-based measures.

Category Templates
Pleasant/Unpleasant (Insects/Flowers) T are A, T is A
Pleasant/Unpleasant (EA/AA) T are A, T is A
Career/Family (Male/Female) T likes A, T like A, T is interested in A
Math/Arts (Male/Female) T likes A, T like A, T is interested in A
Science/Arts (Male/Female) T likes A, T like A, T is interested in A
Table 1: Template sentences used for the WEAT tests (T: target, A: attribute)
Category Targets Templates
Pleasant/Unpleasant (Insects/Flowers) flowers,insects,flower,insect T are A, the T is A
Pleasant/Unpleasant (EA/AA) black, white T people are A, the T person is A
Career/Family (Male/Female) he,she,boys,girls,men,women T likes A, T like A, T is interested in A
Math/Arts (Male/Female) he,she,boys,girls,men,women T likes A, T like A, T is interested in A
Science/Arts (Male/Female) he,she,boys,girls,men,women T likes A, T like A, T is interested in A
Table 2: Template sentences used and target words for the grammatically correct sentences (T: target, A: attribute)
Category WEAT on GloVe WEAT on BERT Ours on BERT
Log Probability Bias Score
Pleasant/Unpleasant (Insects/Flowers) 1.543* 0.6688 0.8744*
Pleasant/Unpleasant (EA/AA) 1.012 1.003 0.8864*
Career/Family (Male/Female) 1.814* 0.5047 1.126*
Math/Arts (Male/Female) 1.061 0.6755 0.8495*
Science/Arts (Male/Female) 1.246* 0.8815 0.9572*
Table 3: Effect sizes of bias measurements on WEAT Stimuli. (* indicates significant at )

3 Correlation with Human Biases

We investigate the correlation between our measure of bias and human biases. To do this, we apply the log probability bias score to the same set of attributes that were shown to exhibit human bias in experiments that were performed using the Implicit Association Test Greenwald et al. (1998). Specifically, we use the stimuli used in the Word Embedding Association Test (WEAT) Caliskan et al. (2017).

Word Embedding Association Test (WEAT): The WEAT method compares set of target concepts (e.g. male and female words) denoted as and (each of equal size ), with a set of attributes to measure bias over social attributes and roles (e.g. career/family words) denoted as and . The degree of bias for each target concept is calculated as follows:

where sim

is the cosine similarity between the embeddings. The test statistics is

where the test is a permutation test over and . The -value is computed as

The effect size is measured as

It is important to note that the statistical test is a permutation test, and hence a large effect size does not guarantee a higher degree of statistical significance.

3.1 Baseline: WEAT for BERT

To apply the WEAT method on BERT, we first compute the embeddings for target and attribute words present in the stimuli using multiple templates, such as “TARGET is ATTRIBUTE” (Refer Table 1 for an exhaustive list of templates used for each category). We mask the TARGET to compute the embedding333We use the outputs from the final layer of BERT as embeddings for the ATTRIBUTE and vice versa. Words that are absent in the BERT vocabulary are removed from the targets. We ensure that the number of words for both targets are equal, by removing random words from the smaller target set. To confirm whether the reduction in vocabulary results in a change of -value, we also conduct the WEAT on GloVe with the reduced vocabulary.444WEAT was originally used to study the GloVe embeddings

3.2 Proposed: Log Probability Bias Score

To compare our method measuring bias, and to test for human-like biases in BERT, we also compute the log probability bias score for the same set of attributes and targets in the stimuli. We compute the mean log probability bias score for each attribute, and permute the attributes to measure statistical significance with the permutation test. Since many TARGETs in the stimuli cause the template sentence to become grammatically incorrect, resulting in low predicted probabilities, we fixed the TARGET to common pronouns/indicators of category such as flower, he, she (Table 2

contains a full list of target words and templates). This avoids large variance in predicted probabilities, leading to more reliable results. The effect size is computed in the same way as the WEAT except the standard deviation is computed over the mean

log probability bias scores.

We experiment over the following categories of stimuli in the WEAT experiments: Category 1 (flower/insect targets and pleasant/unpleasant attributes), Category 3 (European American/African American names and pleasant/unpleasant attributes), Category 6 (male/female names and career/family attributes), Category 7 (male/female targets and math/arts attributes) and Category 8 (male/female targets and science/arts attributes).

3.3 Comparison Results

The WEAT on GloVe returns similar findings to those of Caliskan et al. (2017) except for the European/African American names and pleasant/unpleasant association not exhibiting significant bias. This is due to only 5 of the African American names being present in the BERT vocabulary. The WEAT for BERT fails to find any statistically significant biases at . This implies that WEAT is not an effective measure for bias in BERT embeddings, or that methods for constructing embeddings require additional investigation. In contrast, our method of querying the underlying language model exposes statistically significant association across all categories, showing that BERT does indeed encode biases and that our method is more sensitive to them.

4 Case Study: Effects of Gender Bias on Gendered Pronoun Resolution

Dataset

We examined the downstream effects of bias in BERT using the Gendered Pronoun Resolution (GPR) task Webster et al. (2018). GPR is a sub-task in co-reference resolution, where a pronoun-containing expression is to be paired with the referring expression. Since pronoun resolving systems generally favor the male entities Webster et al. (2018), this task is a valid test-bed for our study. We use the GAP dataset555https://github.com/google-research-datasets/gap-coreference by Webster et al. (2018)

, containing 8,908 human-labeled ambiguous pronoun-name pairs, created from Wikipedia. The task is to classify whether an ambiguous pronoun

in a text refers to entity , entity or neither. There are 1,000 male and female pronouns in the training set each, with 103 and 98 of them not referring to any entity in the sentence, respectively.

Model

We use the model suggested on Kaggle,666https://www.kaggle.com/mateiionita/taming-the-bert-a-baseline inspired by Tenney et al. (2019). The model uses BERT embeddings for , and

, given the context of the input sentence. Next, it uses a multi-layer perceptron (MLP) layer to perform a naive classification to decide if the pronoun belongs to

, or neither. The MLP layer uses a single hidden layer with 31 dimensions, a dropout of 0.6 and L2 regularization with weight 0.1.

Gender Prior Prob. Avg. Predicted Prob.
Male 10.3% 11.5%
Female 9.8% 13.9%
Table 4: Probability of pronoun referring to neither entity in a sentence of GPR

Results

Although the number of male pronouns associated with no entities in the training data is slightly larger, the model predicted the female pronoun referring to no entities with a significantly higher probability ( on a permutation test); see Table 4. As the training set is balanced, we attribute this bias to the underlying BERT representations.

We also investigate the relation between the topic of the sentence and model’s ability to associate the female pronoun with no entity. We first extracted 20 major topics from the dataset using non-negative matrix factorization Lee and Seung (2001) (refer to Appendix for the list of topics). We then compute the bias score for each topic as the sum of the log probability bias score for the top 15 most prevalent words of each topic weighted by their weights within the topic. For this, we use a generic template “[TARGET] are interested in [ATTRIBUTE]” where TARGET is either men or women. Next we compute a bias score for each sample in the training data as the sum of individual bias scores of topics present in the sample, weighted by the topic weights. Finally, we measured the Spearman correlation coefficient to be 0.207 (which is statistically significant with ) between the bias scores for male gender across all samples and the model’s probability to associate a female pronoun with no entity. We conclude that models using BERT find it challenging to perform coreference resolution when the gender pronoun is female and if the topic is biased towards the male gender.

5 Real World Implications

In previous sections, we discussed that BERT has human-like biases, which are propagated to downstream tasks. In this section, we discuss another potential negative impact of using BERT in a downstream model. Given that three quarters of US employers now use social media for recruiting job candidates Segal (2014), many applications are filtered using job recommendation systems and other AI-powered services. Zhao et al. (2018) discussed that resume filtering systems are biased when the model has strong association between gender and certain professions. Similarly, certain gender-stereotyped attributes have been strongly associated with occupational salary and prestige Glick (1991). Using our proposed method, we investigate the gender bias in BERT embeddingss for certain occupation and skill attributes.

Datasets: We use three datasets for our study of gender bias in employment attributes:

Discussion We used the following two templates to measure gender bias:

  • “TARGET is ATTRIBUTE”, where TARGET are male and female pronouns viz. he and she. The ATTRIBUTE are job titles from the Employee Salary dataset, or the adjectives from the Positive and Negative traits dataset.

  • “TARGET can do ATTRIBUTE”, where the TARGETs are the same, but the ATTRIBUTE are skills from the O*NET dataset.

Table 5 shows the percentage of attributes that were more strongly associated with the male than the female gender. The results prove that BERT expresses strong preferences for male pronouns, raising concerns with using BERT in downstream tasks like resume filtering.

Dataset Percentage
Salary 88.5%
Pos-Traits 80.0%
Neg-Traits 78.9%
Skills 84.0%
Table 5: Percentage of attributes associated more strongly with the male gender

6 Related Work

NLP applications ranging from core tasks such as coreference resolution (Rudinger et al., 2018) and language identification Jurgens et al. (2017), to downstream systems such as automated essay scoring Amorim et al. (2018), exhibit inherent social biases which are attributed to the datasets used to train the embeddings Barocas and Selbst (2016); Zhao et al. (2017); Yao and Huang (2017). There have been several efforts to investigate the amount of intrinsic bias within uncontextualized word embeddings in binary Bolukbasi et al. (2016); Garg et al. (2018); Swinger et al. (2019) and multiclass Manzini et al. (2019) settings.

Contextualized embeddings such as BERT Devlin et al. (2019) and ELMo Peters et al. (2018) have been replacing the traditional type-level embeddings. It is thus important to understand the effects of biases learned by these embedding models on downstream tasks. However, it is not straightforward to use the existing bias-exposure methods for contextualized embeddings. For instance, May et al. (2019) used WEAT on sentence embeddings of ELMo and BERT, but there was no clear indication of bias. Rather, they observed counterintuitive behavior like vastly different -values for results concerning gender.

Along similar lines, Basta et al. (2019) noted that contextual word-embeddings are less biased than traditional word-embeddings. Yet, biases like gender are propagated heavily in downstream tasks. For instance, Zhao et al. (2019) showed that ELMo exhibits gender bias for certain professions. As a result, female entities are predicted less accurately than male entities for certain occupation words, in the coreference resolution task. Field and Tsvetkov (2019) revealed biases in ELMo embeddings that limit their applicability across data domains. Motivated by these recent findings, our work proposes a new method to expose and measure bias in contextualized word embeddings, specifically BERT. As opposed to previous work, our measure of bias is more consistent with human biases. We also study the effect of this intrinsic bias on downstream tasks, and highlight the negative impacts of gender-bias in real world applications.

7 Conclusion

In this paper, we showed that querying the underlying language model can effectively measure bias in BERT and expose multiple stereotypes embedded in the model. We also showed that our measure of bias is more consistent with human-biases, and outperforms the traditional WEAT method on BERT. Finally we showed that these biases can have negative downstream effects. In the future, we would like to explore the effects on other downstream tasks such as text classification, and device an effective method of debiasing contextualized word embeddings.

Acknowledgments

This material is based upon work supported by the National Science Foundation under Grant No. IIS1812327.

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Appendix

Topic Id Top 5 Words
1 match,round,second,team,season
2 times,city,jersey,york,new
3 married,son,died,wife,daughter
4 best,award,actress,films,film
5 friend,like,work,mother,life
6 university,music,attended,high,school
7 president,general,governor,party,state
8 songs,solo,song,band,album
9 medal,gold,final,won,world
10 best,role,character,television,series
11 kruse,moved,amy,esme,time
12 usa,trunchbull,pageant,2011,miss
13 american,august,brother,actress,born
14 sir,died,church,song,john
15 natasha,days,hospital,helene,later
16 played,debut,sang,role,opera
17 january,december,october,july,married
18 academy,member,american,university,family
19 award,best,played,mary,year
20 jersey,death,james,king,paul

Table 6: Extracted topics for the GPR dataset