Recent advancements in neural language modelling make it possible to rapidly generate vast amounts of human-sounding text. The capabilities of humans and automatic discriminators to detect machine-generated text have been a large source of research interest, but humans and machines rely on different cues to make their decisions. Here, we perform careful benchmarking and analysis of three popular sampling-based decoding strategiestop-k , nucleus sampling, and untruncated random samplingand show that improvements in decoding methods have primarily optimized for fooling humans. This comes at the expense of introducing statistical abnormalities that make detection easy for automatic systems. We also show that though both human and automatic detector performance improve with longer excerpt length, even multi-sentence excerpts can fool expert human raters over 30% of the time. Our findings reveal the importance of using both human and automatic detectors to assess the humanness of text generation systems. State-of-the-art generative language models are now capable of producing multi-paragraph excerpts that at a surface level are virtually indistinguishable from human-written content (Zellers et al., 2019; Radford et al., 2019; Adelani et al., 2020). Often, only subtle logical fallacies or idiosyncrasies of language give away the text as machine-generated, errors that require a close reading and/or domain knowledge for humans to detect. Deceptive text, whether humanor machine-generated, has entered the sphere of public concern (Cooke, 2018). It propogates quickly (Vosoughi et al., 2018), sets political agendas  Equal contribution,  Google,  University of Pennsylvania (Vargo et al., 2018), influences elections (Allcott and Gentzkow, 2017), and undermines user trust (Wang et al., 2012; Song et al., 2015). Recently, Adelani et al. (2020) have shown that automatically generated reviews are perceived to be as fluent as human-written ones. As generative technology matures, authors, well-meaning or otherwise, will increasingly employ it to augment and accelerate their own writing. It is more imperative now than ever for both humans and automated systems to be able to detect and identify machine-generated texts in the wild. However, there has thus been little inquiry into the textual properties that cause humans to give generated text high human-like ratings compared to those that cause automatic systems to rate it highly. To speak of texts produced by language models, we must first consider how these texts are generated. A neural language model encodes a probability distribution over the next word in a sequence given the previous words. 1 A decoding strategy is an algorithm that generates sequences from a language model by determining how words should get selected from this distribution. The field has largely moved toward probabilistic decoding strategies that randomly sample from the output distribution token-by-token. However, when many low-likelihood words cumulatively contain quite a bit of probability mass, choosing one of these words can lead to odd or contradictory phrases and semantic errors. Humans are quick to notice these types of errors. For this reason, it has become common to modify the language model's output probability distribution to increase the chance of sampling tokens with high likelihood according to the language model. Topk random sampling, where low-likelihood words are restricted from being 1 Often these words are actually subword character sequences such as BPE tokens (Sennrich et al., 2016). generated, is one such method. A language model that is only permitted to produce high-likelihood words is less likely to make a poor choice and create the type of mistakes that are easy for humans to detect. Since humans are not proficient at identifying when a model subtly favors some utterances more often than a human author would, they don't notice the over-representation of high-likelihood words in the generated text. In contrast, automatic systems excel at identifying statistical anomalies and struggle to build deeper semantic understanding. Topk in particular creates text that is easy for machines to detect but very hard for humans. Thus, we observe the general trend: as the number of unlikely words available to be chosen is increased, humans get better at detecting fakes while automatic systems get worse . In this work, we study three popular random decoding strategiestopk , nucleus, and temperature samplingapplied to GPT-2 (Radford et al., 2019). We draw a large number of excerpts generated by each strategy and train a family of BERT-based (Devlin et al., 2019) binary classifiers to label text excerpts as human-written or machine-generated. We find large differences in human rater and classifier accuracy depending on the decoding strategy employed and length of the generated sequences. Regardless of strategy, we find human raters achieve significantly lower accuracy than the automatic discriminators. We also show that when a decoding strategy severely modifies the unigram token distribution, as topk does, humans have trouble detecting the resultant generated text, but automatic classifiers find it the easiest to discriminate. Worryingly, we further find that classifiers are brittle; they generalize poorly when trained to discriminate samples from one strategy and then evaluated on samples from another. In summary, our contributions are:  A comprehensive study of generated text detection systems' sensitivity to model structure, decoding strategy, and excerpt length. An analysis of human raters' ability to identify machine-generated content, and how human raters differ from automatic detectors. Generative Language Models With a suffi-ciently large training set and number of trainable parameters, neural language models based on the Transformer architecture (Vaswani et al., 2017) are capable of generating convincing, human-like excerpts up to several paragraphs in length. GPT-2 (Radford et al., 2019), GROVER (Zellers et al., 2019), and Transformer-DMCA (Liu et al., 2018) are a few examples of large, publicly available models with this ability. GROVER , in particular, has been shown to generate fake news that is more trustworthy than human-written fake news according to human raters. Human Detection The task of trying to guess whether text is coming from a robot or a fellow human was made famous by the Turing Test (Tur-ing, 1950). It continues to be used is chatbot evaluation (Lowe et al., 2017). The related (but not identical) task of asking human raters to judge the quality of machine-generated excerpts remains the gold-standard for evaluating open-domain generation systems (van der Lee et al., 2019). Kreps et al. (2020), Gehrmann et al. (2019), and others have stressed the importance of humans being able to identify fake content on the web. Automatic Detection The rise of machine-generated content has led to the development of automated systems to identify it. GROVER was designed to not only generate convincing news excerpts but to also identify them using a fine-tuned version of the generative model itself (Zellers et al., 2019). GLTR, expecting attackers to use sampling methods that favor high-likelihood tokens, aims to make machine-generated text detectable by computing histograms over per-token log likelihoods (Gehrmann et al., 2019). Bakhtin et al. (2019) frame human-text detection as a ranking task and evaluate their models' cross-domain and cross-model generalization, finding signifi-cant loss in quality when training on one domain and evaluating on another. Schuster et al. (2019) argue that the language distributional features implicitly or explicitly employed by these detectors are insufficient; instead, one should look to explicit fact-verification models. Finally, discriminators for whether text is machine-generated are a promising research direction in adversarial training (Lin et al., 2017; Li et al., 2017) and in automatic evaluation of generative model quality (Novikova et al., 2017; Kannan and Vinyals, 2017; Lowe et al., 2017). Natural Language Understanding Automatic detection of machine-generated text benefits from a semantic understanding of the text. Contradictions, falsehoods, and topic drift can all indicate that an excerpt was machine-generated. Encoder-only Transformer models such as BERT (Devlin et al., 2019) have been shown to do very well at tasks requiring this understanding. While we fine-tune BERT for the task of classifying whether text was machine-generated, others have used the contextual word embeddings from a pre-trained BERT model without fine-tuning to compute a quality score for generated text (Zhang et al., 2020). It is worth noting that recent work has raised questions as to whether BERT truly builds a semantic understanding to make its predictions, or whether it merely takes advantage of spurious statistical differences between the text of different classes (Niven and Kao, 2019). We frame the detection problem as a binary classification task: given an excerpt of text, label it as either human-written or machine-generated. In particular, we are interested in how variables such as excerpt length and decoding strategy impact performance on this classification task. We thus create several datasets. Each is approximately balanced between positive examples of machine-generated text and negative examples of human-written text. While they all share the same human-written examples, each dataset contains a different set of machine-generated examples sampled using one particular decoding strategy. We also build additional datasets by truncating all of the examples to a particular sequence length, By training a separate classifier on each dataset, we are able to answer questions about which decoding strategy results in text that is the easiest to automatically disambiguate from human-written text. We are also able to answer questions about how the length of the examples in the training set impacts our ability to automatically classify excerpts of that same length as either human-written or machine-generated. All of our generated text samples are drawn from GPT-2, a state-of-the-art Transformer-based generative language model that was trained on text from popular web pages (Radford et al., 2019). While we use the GPT-2 LARGE model with 774M parameters, we found that similar trends to those reported here hold in experiments with smaller language models. Given an autoregressive language model that defines a probability distribution over the next token given the previous tokens in a sequence, a decoding strategy generates text by deciding how to output a token at each step based on the predicted distributions. Perhaps the most straightforward decoding strategy is to randomly choose a token with probability proportional to its likelihood. A challenge with the random sampling approach is that these probability distributions often contain a long tail of vocabulary items that are individually low-probability but cumulatively comprise a substantial amount of probability mass. Holtzman et al. (2020) observe that choosing tokens from this tail often leads to incoherent generations. Topk sampling, nucleus sampling, and (in the extreme) beam search have all been proposed to heuristically promote samples with higher per-token likelihoods. Topk and nucleus sampling both do so by setting the likelihood of tokens in the tail of the distribution to zero. Topk restricts the distribution to all but the k most likely tokens, where k is a constant (Fan et al., 2018). Nucleus sampling, also called topp , truncates the distribution at each decoding step t to the k t -most-likely next tokens such that the cumulative likelihood of these tokens is no greater than a constant p (Holtz-man et al., 2020). We thus consider three different decoding strategy settings:  Sample from the untruncated distribution  Topk , choosing k =40 (Radford et al., 2019). Nucleus sampling (aka topp ), choosing p =0.96 (Zellers et al., 2019). In addition, we form negative examples of human-written text by taking excerpts of web text that come from the same distribution as GPT-2's training data. 2 By picking text that resembles GPT-2's train set, we ensure that our classifiers can't simply take advantage of stylistic differences between the human-written text corpus and the kind of text GPT-2 was trained to generate. For each decoding method, we construct a training dataset by pairing 250,000 generated samples with 250,000 excerpts of web text. 5,000 additional paired samples are kept aside for validation and test datasets. Lastly, we filter out excerpts with fewer than 192 WordPiece tokens (Wu et al., 2 https://github.com/openai/ gpt-2-output-dataset 2016) (excerpts might be quite short if the model produces an end-of-text token early on). See Appendix 1 for final dataset sizes. A crucial question when generating text with a language model is whether or not to provide a priming sequence which the language model should continue. Unconditioned samples, where no priming text is provided, in conjunction with topk sampling, lead to pathological behavior for discriminators as the first token of the generated text will always be one of k possible options. On the other hand, if long sequences of human text are used as priming, the space of possible generated sequences is larger, but the detection problem shifts from one of how human-like is the generated text? to how well does the generated text follow the priming sequence?. Since in this study we are interested in the former simpler question, we create two datasets, one with no priming, and one with the minimum amount of priming possible: a single token of web text. This means that for every excerpt of web text in the training set, there is an excerpt of machine-generated text that starts with the same token. We find that even with limited priming, the ability of automatic detectors can be strongly impacted. To study the effect of excerpt length, we construct variations of the above datasets by truncating all excerpts to ten possible lengths ranging from 2 to 192 WordPiece tokens (Wu et al., 2016). In total, we obtain sixty dataset variations: one per sampling method, truncation length, and choice of priming or no priming. The primary discriminator we employ is a finetuned BERT classifier (Devlin et al., 2019). We fine-tune one instance of BERT per dataset variation described above. For the longest sequence length, n =192, we compare BERT's performance with several simple baselines that have been proposed in other work. Fine-tuned BERT We fine-tune BERT-LARGE (cased) on the task of labeling a sentence as humanor machinegenerated. The models are trained for 15 epochs, with checkpoints saved every 1000 steps, and a batch size of 256. All results are reported on the test set using the checkpoint for which validation accuracy was highest. Bag-of-Words For each sequence, we compute a bag-of-words embedding where each dimension corresponds to a token in GPT-2's 50,000 token BPE vocabulary (Sennrich et al., 2016), and we count how many times that token appears in the text sequence. We then train a logistic regression binary classifier to predict humanor machine-written given this 50,000-dimensional embedding. We experimented with truncating embedding size by removing entries for infrequent vocabulary words, but this did not improve performance. Histogram-of-Likelihood Ranks Following GLTR (Gehrmann et al., 2019), we compute the probability distribution of the next word given the previous words in a text sequence according to a trained language model (in our case the same GPT-2 model that was used for generation). At each sequence position, we rerank the vocabulary words by likelihood, and record the rank of the ground-truth next word within this list. These ranks are then binned. GLTR uses four bins, counting (1) the number of times the top 1 word is seen, (2) the number of times words ranked 2 through 5 are seen, (3) words ranked 6-100, and (4) words ranked > 100. However, we observe higher accuracy when 50 bins are spread uniformly over the possible rankings. This means that since there are 50,000 vocabulary words, the first bin counts the number of times the actual next word was within the 1,000 mostly likely next words, the second bin counts the 1,001-2,000th, and so on. We then train logistic regression binary classifiers to predict humanor machine-written given either the 4-dimensional histograms or 50-dimensional histograms as input. Total Probability Solaiman et al. (2019) propose a very simple baseline consisting of a threshold on the total probability of the text sequence. An excerpt is predicted as machine-generated if its likelihood according to GPT-2 is closer to the mean likelihood over all machine-generated sequences than to the mean of human-written ones. The human evaluation task is framed similarly to the automatic one. We ask the raters to decide whether a passage of text was written by a human or by a computer algorithm. (Full instructions are in the Appendix.) Raters are allowed to choose between four options: definitely or possibly machine-generated and definitely or possibly human-written. They are first shown an excerpt of length 16 WordPiece tokens. After they make BERT BagOfWords HistGLTRBuckets Hist50Buckets TotalProb Human Method acc AUC acc AUC acc AUC acc AUC acc acc k40-1wordcond 0.88 0.99 0.79 0.87 0.52 0.52 0.69 0.76 0.61 0.64 p0.96-1wordcond 0.81 0.89 0.60 0.65 0.53 0.56 0.54 0.56 0.63 0.77 p1.0-1wordcond 0.79 0.92 0.59 0.62 0.53 0.55 0.54 0.55 0.65 0.71 Table 1: Performance (accuracy and AUC) of the fine-tuned BERT classifier and several simple baselines on detecting length-192 sequences generated with one word of priming (1worccond). a guess, the length of the excerpt is doubled, and they are asked the same question again. This continues until the entire passage of length 192 tokens is shown. Passages are equally likely to be human-written or machine-generated, with the machine-generated excerpts being evenly split between the three sampling strategies considered in this paper. Initially, Amazon Mechanical Turk (AMT) raters were employed for this task, but rater accuracy was poor with over 70% of the definitely votes cast for human despite the classes being balanced. Accuracy, even for the longest sequences, hovered around 50%. The same study was then performed with university students who were first walked through ten examples (see Appendix Table 4) as a group. Afterward, they were asked to complete the same tasks that had been sent to the AMT workers. No additional guidance or direction was given to them after the initial walk-through. We will refer to this group as the expert raters. Among them, 52.1% of def-initely votes were cast for human, and accuracy on the longest excerpt length was over 70%. The human evaluation dataset consisted of 150 excerpts of web text and 50 excerpts each from the three decoding strategies. Each question was shown to at most three raters, leading to 900 total annotations from the untrained workers and 475 from the expert raters. A more detailed breakdown can be found in the Appendix. Simple Baselines Table 1 shows the performance of the baseline discriminators on length-192 sequences, as compared with fine-tuned BERT. Reassuringly, BERT far surpasses all simple baselines, indicating that it is not fully possible to solve the detection problem without complex sequence-based understanding. The simplest baseline, TotalProb, which makes a decision based on the likelihood of the sequence, performs surprisingly well (over 60% accuracy for all sampling methods) relative to the methods which involve training logistic regression models. Logistic regression on bag-of-words is the best of the baselines, beating out the histogram-based methods. While Gehrmann et al. (2019) report an AUC of 0.87 on classifying text as real or generated using logistic regression on the four buckets of the GLTR system, we report AUC between 0.52 and 0.56 for this task. The discrepancy is likely due to the fact that the human-written text in our discriminator training set comes from the same distribution as the text used to train the language model, while in GLTR the human text comes from children's books, scientific abstracts, and newspaper articles. The selection of training data for learned detection systems is crucial. In real-world applications, the choice ought to reflect the genres that builders of text-generation systems are trying to impersonate. Fine-tuned BERT In Figure 1a, we begin by observing discriminator accuracy as a function of excerpt length and sampling method. As can be intuitively expected, as sequence length increases, so too does accuracy. For unconditioned text decoded with nucleus (p0.96) and untruncated (p1.0) random sampling, we find discriminator accuracy increases from 55%, near random, to about 81% for the longest sequences tested. In contrast, discriminators trained and evaluated on topk achieve over 80% accuracy even on 16-token excerpts. Why are topk 's samples so easy to detect? In Figure 2b, we see the percentage of probability mass concentrated in the k most common token types for each sampling method. While random sampling and nucleus sampling are very similar to human-written texts, we see top-k concentrating up to 80% of its mass in the first 500 most common tokens. The other sampling methods as well as human-written texts require at least 1,100 token types for the same. It is clear that topk 's distribu-50% 55% 60% 65% 70% 75% 80% 85% 90% 95% 100% 0 32 64 96 128 160 192 A cc u r a cy Sequence length in tokens Accuracy of BERT Fine-tuned Discriminator k40-1wordcond k40-nowordcond p0.96-1wordcond p0.96-nowordcond p1.0-1wordcond p1.0-nowordcond tion over unigrams strongly diverges from human-written textsan easy feature for discriminators to exploit. In fact, See et al. (2019) note that it takes setting k to 1000 to achieve about the same amount of rare word usage and fraction of non-stopword text as as human writing. 3 This makes it very easy for the model to pick out machine-generated text based on these distributional differences. One way to help resolve this problem is to add priming text. Doing so causes more rare words to be incorporated into the topk of the unigram distribution. Adding even a single human word of priming significantly reduces the performance of detectors trained with topk random sampling. Without priming, a discriminator trained on sequences of length 2 can classify with  90% accuracy the provenance of the text (Figure 1a). By adding one priming token, accuracy drops to  65%. Even on the longest 192-length sequences, topk discriminator accuracy is 6% lower on the primed dataset than the unprimed one. When generating with nucleus or untruncated random sampling, adding a priming token is not as impactful, as these methods are already sampling from a large fraction (or all) of the probability distribution. This is seen in Figure 2a where at the very first step of unprimed generation, nucleus sampling selects from 3075 possible vocabulary words, and at later positions selects from on 3 when decoding from the GPT-2 small model with 117M parameters. Transferability In Table 2, we show how discriminators trained with samples from one decoding strategy can transfer at test time to detecting samples generated using a different decoding strategy. Unsurprisingly a discriminator trained on topk generalizes poorly to other sampling methods: accuracy drops to as low as 42.5%, worse than chance . Conversely, training the discriminator with sequences sampled from the untruncated distribution leads to little transferability to detecting topk samples. Only the discriminator trained with nucleus sampling (a compromise between unmodified sampling and topk ) was able to detect sequences from the other sampling strategies without too much of a hit to accuracy. As expected, a discriminator trained on an equal portion of data from each decoding method does reasonably at detecting all three. Perhaps this lack of transferability is related to each discriminator's calibration. Indeed, the degree to which a discriminator's average prediction deviates from 50% is a direct indicator of its accuracy. In Table 3, we observe that of the three BERT discriminators, only that trained on topp samples predicts machine-generated' on approximately 50% of in-domain examples as expected. This same discriminator's behavior holds on datasets generated by other sampling strategies 0 50 100 150 200 Position in sequence 500 1000 1500 2000 2500 3000 3500 4000 4500 k Mean k Chosen at each Position during Generation with Nucleus Sampling p0.96-nowordcondp0.96-1wordcond as well. In contrast, we observe that discriminators trained on top-k and untruncated random samples severely underestimate the percentage of machine-generated excerpts in out-of-domain datasets. Even within domain (Figure 1b), we find both discriminators heavily favor a single class, in-Eval topk nucleus random T r a i n top-k 60.9 27.9 14.5 nucleus 49.2 51.7 48.9 random 7.3 22.6 38.3 Table 3: Average probability of machine-generated' according to each length-192 discriminator. Human Evaluation Overall human performance across all sampling methods is shown in Figure 3b. Even with the multi-paragraph 192-length excerpts, human performance is only at 71.4%, indicating that even trained humans struggle to correctly identify machine-generated text over a quar-Truth Raters p1.0 k40 p0.96 Truth Raters p1.0 k40 p0.96 H M H H M H H M M M ter a time. However, it is worth noting that our best raters achieved accuracy of 85% or higher, suggesting that it is possible for humans to do very well at this task. Further investigation is needed into how educational background, comfort with English, participation in more extensive training, and other factors can impact rater performance. To break up the accuracies by sampling method in a way that is comparable to the results shown for the automatic discriminators, we pair each machine-generated example with a randomly selected one of webtext to create a balanced dataset for each sampling strategy. Performance is shown in Figure 3a. Topk produces the text that is hardest for raters to correctly distinguish, but as shown in Section 7, it is the easiest for our automatic detection systems. Samples from untruncated random sampling and nucleus sampling with p =0.96 are equivalently difficult for raters to classify as machine-generated. Our human evaluation results suggest that much lower p -values than the 0.92 to 0.98 range proposed in Zellers et al. (2019) might be necessary in order to generate text that is considered significantly more human-like to human raters than the text produced by using the untruncated distribution. Table 4 gives several examples where human raters and our BERT-based discriminators disagreed. When raters incorrectly labeled human-written text as machine-generated, often the excerpts contained formatting failures introduced when the HTML was stripped out. In the mid-dle two examples, topic drift and falsehoods such as Atlanta being the information hub of the na-tion's capital allowed humans to correctly detect the generated content. However, in the bottom two examples, the high level of fluency left human raters fooled. Overall we find that human raterseven expert trained oneshave consistently worse accuracy than automatic discriminators for all decoding methods and excerpt lengths. In our experiments, randomly-selected pairs of raters agree with each other on a mere 59% of excerpts on average. (In comparison, raters and discriminators agree on 61% to 70% of excerpts depending on the discriminator considered). We surmise that the gap between human and machine performance will only grow as researchers inevitably train bigger, better detection models on larger amounts of training data. While improved detection models are inevitible, it is unclear how to go about improving human performance. GLTR proposes providing visual aids to humans to improve their performance at detecting generated-text, but it is unlikely that their histogram-based color-coding will continue to be effective as generative methods get better at producing high-quality text that lacks statistical anomalies. In this work, we study the behavior of automated discriminators and their ability to identify machine-generated and human-written texts. We train these discriminators on balanced binary classification datasets where all machine-generated excerpts are drawn from the same generative model but with different decoding strategies. We find that, in general, discriminators transfer poorly between decoding strategies, but that training on a mix of data from methods can help. We also show the rate at which discriminator accuracy increases as excerpts are lengthened. We further study the ability of expert human raters to perform the same task. We find that rater accuracy varies wildly, but has a median of 74%, which is less than the accuracy of our best-performing discriminator. Most interestingly, we find that human raters and discriminators make decisions based on different qualities, with humans more easily noticing semantic errors and discriminators picking up on statistical artifacts. In our experiments, these artifacts are most prominent with topk sampling. However, any strategy that over-samples high-likelihood words is susceptible. As the p in nucleus sampling is set increasingly lower to achieve more fluent text (some systems are already using p as low as 0.5 (Miculicich et al., 2019)), the distributional deviations that plague topk text will surface in nucleus sampling as well. Holtzman et al. (2020) explain how a unique attribute of human language is that it dips in and out of low probability zones. This variance in likelihood is what makes human-written text interesting and exciting to read. Today's generation systems have not yet solved the problem of mimicking the human cadence without introducing poor word choices that are easy for humans to detect. Generation systems often optimize for fooling humans without acknowledging the trade-off that exists between human perception of quality and ease of automatic detection. We therefore suggest three prongs for future research: 1. Identifying ways to improve the language models and decoding strategies we use in order to generate text that is both exciting (ie. unlikely) and semantically plausible. 2. Building better world understanding into automatic discriminators so that they are more capable of detecting the types of errors that humans notice. 3. Developing tools and educational materials to improve humans' ability to detect machine-generated text. These may include automatic detectors with components that explain their predictions. Finally, we would like to note that all of our experiments were performed with English language models, and it remains an open question how the trade-off between ease of human detection and ease of automatic detection might differ for languages that are very different from English. This research is based upon work supported in part by U.S. DARPA KAIROS Program No. FA8750-19-2-1004. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of DARPA or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright annotation therein. We also thank Noah Fiedel, Peter Liu, Sharan Narang, Joao Sedoc, Yun William Yu, and Hugh Zhang for their valuable feedback.
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