Do AI voices follow social nuances? The case of politeness and speech rate

Voice-based artificial intelligence is increasingly expected to adhere to human social conventions, but can it exhibit implicit cues that are not explicitly programmed? This study investigates whether state-of-the-art text-to-speech systems have internalized the human tendency to reduce speech rate to convey politeness - a non-obvious prosodic marker. We prompted 22 synthetic voices from two leading AI platforms (AI Studio and OpenAI) to read a fixed script under both "polite and formal" and "casual and informal" conditions and measured the resulting speech duration. Across both AI platforms, the polite prompt produced slower speech than the casual prompt with very large effect sizes, an effect that was statistically significant for all of AI Studio's voices and for a large majority of OpenAI's voices. A second study confirmed that these prosodic adjustments are perceptually salient to human listeners, who successfully distinguished between the intended polite and casual styles based on the AI’s output. These results demonstrate that AI can implicitly replicate the statistical patterns of human communication, highlighting its emerging role as a social actor that can reinforce human social norms.