Many prehistoric societies have left a wealth of inscribed symbols for which the meanings are lost. For example, the Picts, a Scottish, Iron Age culture, left a few hundred stones expertly carved with highly stylized petroglyph symbols. Although the symbol scripts are assumed to convey information, owing to the short (one to three symbols), small (less than 1000 symbols) and often fragmented nature of many symbol sets, it has been impossible to conclude whether they represent forms of written language. This paper reports on a two-parameter decision-tree technique that distinguishes between the different character sets of human communication systems when sample sizes are small, thus enabling the type of communication expressed by these small symbol corpuses to be determined. Using the technique on the Pictish symbols established that it is unlikely that they are random or sematographic (heraldic) characters, but that they exhibit the characteristics of written languages. © 2010 The Royal Society.