What language do deaf people think in?

Thanks to Bencie Woll and Victoria Mousley for the answer!

Will - Profound deafness is when an individual can hear nothing, save for occasionally extremely loud sounds. So if you do not hear sounds, including those of people and yourself talking, what form do your thoughts take? Well, that depends on when the individual became profoundly deaf. As deafness cognition expert at UCL Bencie Woll explains...

Bencie - Somebody who's become deaf late in life and who grew up only speaking English, their dreams are going to be in English. A person who was born deaf, the languages they might think in are going to be related to the languages they know. You can see young deaf children signing to themselves just the way young hearing children speak to themselves.

Will - So if your deafness came on in later life, it's highly likely you'll retain the language you heard growing up as the one you think in. But those born deaf will see or even feel themselves signing in their head.

Bencie - And the most interesting research is on deaf people with schizophrenia. People who had some hearing in their lives but became deaf later. They're perfectly able to imagine just a voice. People who were born deaf do report voices, hearing voices being in conversation, but there's always an image of a face or a person to go with it because that's their experience of spoken language that you only communicate when you can see a person.

Will - Fascinating stuff, but with this being a language same as any other, does it develop in the same way as spoken language among those profoundly deaf at birth? Here to explain that is Beck, University of London's Victoria Mousley.

Victoria - We know from research with hearing children that younger kids, more so than older kids are likely to use and to benefit from overt self-talk during certain tasks, so talking their way through certain things. It seems like after about five years of age, kids do this less and it's also less useful for them. So this could be around the time where we start to see the development of covert self-talk or inner voices in young children. We also know that profoundly deaf kids who experience full native language access early in life achieve very similar cognitive and linguistic milestones to hearing children. So I would expect that deaf and hearing kids would develop inner voices at pretty similar ages, and that both deaf and hearing kids inner voices likely reflect their language and communicative experiences up until that point. But we would need more research to test these hypotheses.