The Puss in Boots effect

Pet-directed speech (PDS) is often produced by humans when addressing dogs. Similar to infant-directed speech, PDS is marked by a relatively higher and more modulated fundamental frequency (f(0)) than is adult-directed speech. We tested the prediction that increasing eye size in dogs, one facial feature of neoteny ( juvenilisation), would elicit exaggerated prosodic qualities or pet-directed speech. We experimentally manipulated eye size in photographs of twelve dog breeds by -15%, +15% and +30%. We first showed that dogs with larger eyes were indeed perceived as younger. We then recorded men and women speaking towards these photographs, who also rated these images for cuteness. Linear mixed-effects models demonstrated that increasing eye size by 15% significantly increased pitch range (f(0) range) and variability (f(0)CV) among women only. Cuteness ratings did not vary with eye size, due to a possible ceiling effect across eye sizes. Our results offer preliminary evidence that large eyes can elicit pet-directed speech and suggest that PDS may be modulated by perceived juvenility rather than cuteness. We discuss these findings in the context of inter-species vocal communication.