Hallucination in Large Language Models (LLMs) and Its Causes

The emergence of large language models (LLMs) such as Llama, PaLM, and GPT-4 has revolutionized natural language processing (NLP), significantly advancing text understanding and generation. However, despite their remarkable capabilities, LLMs are prone to producing hallucinations, content that is factually incorrect or inconsistent with user inputs. This phenomenon substantially challenges its reliability in real-world applications, necessitating a comprehensive understanding of its principles, causes, and mitigation strategies.

Definition and Types of Hallucinations

Hallucinations in LLMs are typically categorized into two main types: factuality hallucination and faithfulness hallucination.

  1. Factuality Hallucination: This type involves discrepancies between the generated content and verifiable real-world facts. It is further divided into:
  • Factual Inconsistency: Occurs when the output contains factual information that contradicts known facts. For instance, an LLM might incorrectly state that Charles Lindbergh was the first to walk on the moon instead of Neil Armstrong.
  • Factual Fabrication: Involves the creation of entirely unverifiable facts, such as inventing historical details about unicorns.
  1. Faithfulness Hallucination: This type refers to the divergence of generated content from user instructions or the provided context. It includes:
  • Instruction Inconsistency: When the output does not follow the user’s directive, such as answering a question instead of translating it as instructed.
  • Context Inconsistency: Occurs when the generated content contradicts the provided contextual information, such as misrepresenting the source of the Nile River.
  • Logical Inconsistency: Involves internal contradictions within the generated content, often observed in reasoning tasks.

Causes of Hallucinations in LLMs

The root causes of hallucinations in LLMs span the entire development spectrum, from data acquisition to training and inference. These causes can be broadly categorized into three parts:

1. Data-Related Causes:

  • Flawed Data Sources: Misinformation and biases in the pre-training data can lead to hallucinations. For example, heuristic data collection methods may inadvertently introduce incorrect information, leading to imitative falsehoods.
  • Knowledge Boundaries: LLMs may lack up-to-date factual or specialized domain knowledge, resulting in factual fabrications. For instance, they might provide outdated information about recent events or need more expertise in specific medical fields.
  • Inferior Data Utilization: LLMs can produce hallucinations due to spurious correlations and knowledge recall failures even with extensive knowledge. For example, they might incorrectly state that Toronto is the capital of Canada due to the frequent co-occurrence of “Toronto” and “Canada” in the training data.

2. Training-Related Causes:

  • Architecture Flaws: The unidirectional nature of transformer-based architectures can hinder the ability to capture intricate contextual dependencies, increasing the risk of hallucinations.
  • Exposure Bias: Discrepancies between training (where models rely on ground truth tokens) and inference (where models rely on their outputs) can lead to cascading errors.
  • Alignment Issues: Misalignment between the model’s capabilities and the demands of alignment data can result in hallucinations. Moreover, belief misalignment, where models produce outputs that diverge from their internal beliefs to align with human feedback, can also cause hallucinations.

3. Inference-Related Causes:

  • Decoding Strategies: The inherent randomness in stochastic sampling strategies can increase the likelihood of hallucinations. Higher sampling temperatures result in more uniform token probability distributions, leading to the selection of less likely tokens.
  • Imperfect Decoding Representations: Insufficient context attention and the softmax bottleneck can limit the model’s ability to predict the next token, leading to hallucinations.

Mitigation Strategies

Various strategies have been developed to address hallucinations, improve data quality, enhance training processes, and refine decoding methods. Key approaches include:

  1. Data Quality Enhancement: Ensuring the accuracy and completeness of training data to minimize the introduction of misinformation and biases.
  2. Training Improvements: Developing better architectural designs and training strategies, such as bidirectional context modeling and techniques to mitigate exposure bias.
  3. Advanced Decoding Techniques: Employing more sophisticated decoding methods that balance randomness and accuracy to reduce the occurrence of hallucinations.


Hallucinations in LLMs present significant challenges to their practical deployment and reliability. Understanding hallucinations’ various types and underlying causes is crucial for developing effective mitigation strategies. By enhancing data quality, improving training methodologies, and refining decoding techniques, the NLP community can work towards creating more accurate and trustworthy LLMs for real-world applications.


  • https://arxiv.org/pdf/2311.05232

Sana Hassan, a consulting intern at Marktechpost and dual-degree student at IIT Madras, is passionate about applying technology and AI to address real-world challenges. With a keen interest in solving practical problems, he brings a fresh perspective to the intersection of AI and real-life solutions.

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