The study was concerned with investigating the impact of social presence on the private speech of participants. Previous research has found that natural interaction and scaffolding with a participant can either decrease or increase the production of private speech respectively. The current study involved no interaction between the participants and social other to focus on the effect the presence of another has on private speech. In addition to this, age trends and the role of private speech were measured to detect a developmental pattern and a self-regulatory function from the theories set out by Piaget (1923) and Vygotsky (1934). To do this, 24 children aged three to five years, and 24 adults were required to solve two classification tasks on the computer; one task in isolation, and one in the company of another. The frequency and content of private speech utterances were measured for all participants. From this, the current study found that all participants produced more private speech when in the company of another compared with when in isolation. A developmental trend of private speech across the lifespan was detected, with children producing significantly more private speech than adults. From the inspection of the content of private speech, a self-regulatory function was apparent for all participants. A model has been created to explain the outcomes from this experiment, showing the process from inner thought to speech via a social presence and taboo filter.