Can you trust AI to pick the best stocks?

There have been a series of papers published recently that showed that large language models have their own biases when making investment recommendations. I have previously written about the kind of biases displayed, while this study has looked at the particular biases of different commonly used models. Fair enough, but what if I tell you that one can manipulate the investment bias of an open-weight LLM by changing a single neuron in the neural network? That is the concerning message of a new paper by a group of researchers from South Korea and the US. They started with popular open-weight models (Qwen3, Llama 4, DeepSeek R1, Gemma 4, and Mistral Small) and identified a single neuron in each of these models that would influence the investment bias of the model output. By changing the activation of this single neuron in the one-weight model, they could switch the investment recommendation for single stocks or entire portfolios without modifying the prompt or updating m