A large-scale study covering 208,000 participants and 26 million responses shows that the very training that turns language models into helpful chatbots weakens their ability to replicate human behavior. The effect gets worse with each new model generation. Even the popular persona trick, feeding models demographic profiles, brings practically no benefit for individual predictions. A large-scale study shows that the training process turning raw language models into helpful chatbots also weakens their ability to mimic human behavior. The effect gets worse with each new generation. Language models are increasingly used as stand-i... [5564 chars]