Most people take for granted that the organs we use to speak, such as the tongue, lips, teeth and the bony roof of the mouth (hard palate), somehow emerge during development and then pretty much “quietly” do their job. However, on a closer inspection (or a chat with your dentist while the anaesthetic kicks in) will soon reveal that they vary tremendously, in more or less subtle ways, between “normal” people: some have longer tongues that can touch the tip of their nose, some have smaller teeth, and yet some have high, domed hard palates.
Does this variation matter for speech, and where does it come from? To the first question, the answer suggested by recent studies is that, at least in some cases, it might. To the second, the answer is largely unknown given the complexity of the interplay between genes, environment and culture, and the lack of large scale-studies using adequate methodology and data.
To remedy this, an international consortium, initiated by Dan Dediu and Dorret Boomsma, has combined 3D structural magnetic resonance imaging (MRI) of the speech organs from more than 600 identical and non-identical twins. These data were collected across almost two decades and five different studies at the Netherlands Twin Register, Vrije Universiteit Amsterdam, and allowed the team an in-depth analysis of the interplay between genes and environment for 140 aspects of the vocal tract. This interplay is captured by their so-called heritability which can vary between 0% and 100%, and expresses the proportion of the observed variation between individuals due to variation in their genes.
The results are complex and suggest that the interplay between genes and environment varies dramatically between components of the vocal tract, from the unexpectedly low heritability of the hard palate to the unexpectedly high heritability of the position of the voice box (larynx). Overall, the results are consonant with observations that cultural practices (including the food consumed) do matter. The study opens new directions in the “hunt” for the genes and environmental and cultural factors at play by focusing future research, with potential therapeutic applications.