FIVE years ago three well-known academics, including Noam Chomsky, wrote that the half-century old “interdisciplinary marriage” between biology and linguistics “has not yet been fully consummated.” That same year other scientists described the molecular evolution of a gene called FOXP2 which, when mutated, seems to cause people severe difficulty with grammar and articulation.

Another genetic condition that could shed light on the biology of linguistics is microcephaly (sometimes rudely called “pin-headedness”). It is linked to six genes, a spanner in the works of any of which leads the human brain to grow to only two-thirds of a pint in adults. That is less than a third of its normal volume. Those genes are alluring objects for studying the evolution of language because brain size has ballooned in people since their line split with that of their closest relatives. Even though birds sing and bees dance, nothing in nature matches a human's richly complicated system of vocal communication. In short, language makes humans unique and genes active in the developing brain make language possible.

Dan Dediu and Robert Ladd, of the University of Edinburgh, are helping biology and linguistics become intimate at the genetic level. In the current issue of the Proceedings of the National Academy of Sciences they describe a statistical analysis of two of the genes that cause microcephaly when they go wrong. Like many genes, both exist in different functional versions, called alleles, which are spread unevenly among human populations.

First, Dr Dediu and Dr Ladd checked that their two genes of interest really are unusual. They combined a database of 983 alleles that are known to vary across human groups with another enumerating how 26 discrete linguistic features (such as whether consonants aggregate at the beginnings and ends of words) are either used or not used in conversation by 49 populations. Picking apart a thicket of possible correlations, Dr Dediu and Dr Ladd found no general links between human genetic and linguistic characters.

But one feature—whether a language uses pitch as well as vowels and consonants to convey word meanings—stood apart. Those, such as Chinese, that encipher meaning in pitch are called “tonal languages”. Those that do not, like English, are “non-tonal”. And it was versions of Dr Dediu's and Dr Ladd's two microcephaly-related genes that matched the 49 populations along tonal and non-tonal lines.

Dr Dediu and Dr Ladd believe the evolution of tonal and non-tonal languages interacted with the evolution of these genes. Certain alleles could have predisposed people to a tonal-language structure. That tonal language, if used by individuals with whom communicating well is particularly helpful, could then reinforce selection for those alleles. Probably not coincidentally, the two alleles that are associated with non-tonal languages evolved recently in human history (some 5,800 and 37,000 years ago) and show signs of being strongly beneficial to their carriers.

Correlations such as these can shift the balance of evidence, but it is demonstrating causation in addition to correlation that forms the glue of the scientific method. Here the marriage between language and genetics hits a problem. No suspicious-looking gene can be added or eliminated in an experimental mouse in order to discover its effects on language, since mice cannot speak. At best, researchers can try to fill in gaps between molecular biology and behaviour.

In March, Narly Golestani and her colleagues at University College, London, showed that people who are fast at learning the sounds of foreign speech have more densely packed white matter in a part of the brain called the left Heschl gyrus. Patrick Wong and his colleagues at the University of Texas, Austin, recently found that adult English speakers fall into two groups, according to their ability to learn tonal distinctions. The more successful also had bigger left Heschl gyruses. The next task is to see if genetic differences can be pinned on the two groups—and, if so, exactly which genes are involved.