FIRST, tap your forehead with your index finger. Then, with hands at shoulder width and forming a circle using the thumb and index finger of each, move your hands from neck height downward, stopping abruptly. If you are not fluent in American Sign Language, you would struggle to guess that this motion means “decide”—but a study published this week suggests that non-signers can guess at least one crucial aspect of the word.

“Decide” is what is known as a telic verb—that is, it represents an action with a definite end. By contrast, atelic verbs such as “negotiate” or “think” denote actions of indefinite duration. The distinction is an important one for philosophers and linguists. The divide between event and process, between the actual and the potential, harks back to the kinesis and energeia of Aristotle’s metaphysics.

One question is whether the ability to distinguish them is hard-wired into the human brain. Academics such as Noam Chomsky, a linguist at the Massachusetts Institute of Technology, believe that humans are born with a linguistic framework onto which a mother tongue is built. Elizabeth Spelke, a psychologist up the road at Harvard, has gone further, arguing that humans inherently have a broader “core knowledge” made up of various cognitive and computational capabilities.

Exploring these ideas is tricky, not least because newborns hold the answer. But sign languages, just as complex and expressive as their spoken counterparts, may give hints by bringing the conceptual into the visual domain.

Ponder and decide

In 2003 Ronnie Wilbur, of Purdue University, in Indiana, noticed that the signs for telic verbs in American Sign Language tended to employ sharp decelerations or changes in hand shape at some invisible boundary, while signs for atelic words often involved repetitive motions and an absence of such a boundary. Dr Wilbur believes that sign languages make grammatical that which is available from the physics and geometry of the world. “Those are your resources to make a language,” she says. As such, she went on to suggest that the pattern could probably be found in other sign languages as well.

Work by Brent Strickland, of the Jean Nicod Institute, in France, and his colleagues, just published in the Proceedings of the National Academy of Sciences, now suggests that it is. Dr Strickland has gone some way to showing that signs arise from a kind of universal visual grammar that signers are working to.

Dr Strickland’s team recruited volunteers from Mechanical Turk, an online small-jobs marketplace run by Amazon. These volunteers reported having no prior experience of sign languages. In the first experiment, they were shown videos of a series of signs from Italian Sign Language. For each, they were asked to guess the sign's meaning and given a pair of options: one telic, and one atelic describing a different kind of action altogether (one pair might, for example, be “forget” and “negotiate”).

Participants guessed correctly more than 90% of the time. To control for the possibility that the the particular type of action may have been a hint that distinguished it, more volunteers were given telic/atelic pairs describing similar actions, such as “forget” and “ponder”. This time, the respondents spotted 85% of the correct answers.

This pattern was not confined to Italian Sign Language. Something similar held when the experimenters ran the same tests using signs from Turkish Sign Language and Sign Language of the Netherlands. Borrowing ideas from Dr Wilbur’s work about the motions characteristic of telic and atelic verb signs, the team even invented nine of each from scratch: respondents accurately spotted the correct, sham ones 64% of the time.

All of this, Dr Strickland says, challenges the long-standing notion in linguistics that the relation between a symbol and its meaning is arbitrary. If the various symbols for “decide”, encoded in a number of different sign languages of different descent, all share unconscious visual cues, then perhaps the relation is not entirely arbitrary after all.

That may be an indication of “core knowledge” that would not surprise the Chomskyists. What might provide more compelling evidence are experiments in a similar vein carried out among speakers of Nicaraguan Sign Language (ISN). This developed spontaneously, in the 1970s, among deaf Nicaraguan schoolchildren: an untainted expression of communication made visual. Dr Strickland expects to find that telic/atelic differences in ISN signs among first-generation speakers will seem slight, and that the distinction will have become sharper as the language has evolved (there is now a third ISN-speaking generation).

The notion that humans may share such communication basics is also fuel to Dr Wilbur’s idea that sign language long predated the spoken kind. Establishing either proposition will take far more work. But the present findings are a good sign.