However, proving a genuine correspondence between artificial laboratory-generated codes and real-world scripts presents at least three three challenges: 1) the property of iconicity that semiotic experiments have focused on is hard to capture and measure given the cross-cultural diversity of figurative traditions and their context-specific perceptions of verisimilitude; and even within a single visual tradition there is no robust way of determining whether a given sign is more or less iconic than another; 2) not all writing systems are observed to lose their iconic or pictographic character; e.g., despite the early emergence of a cursive Hieratic style, Egyptian hieroglyphic signs retained their essential iconic forms over several millennia while ancient Mesoamerican scripts are even seen to have gained iconicity (Mignolo 1989; Downey 2014). 3) Writing has been independently invented only four times in circumstances that are not well understood meaning that earliest phases of script invention have not survived or cannot be recovered. Furthermore, there are precious few independent script lineages to compare, making Galton's problem harder to unravel (Morin under review). These three problems are addressed below.
We argue that the tendency for graphic images in transmission experiments to become perceptibly less iconic over multiple generations is secondary to a more fundamental transition from less compressible to more compressible. That is to say, they become optimised for efficiency in terms of information storage, retrieval and reproduction by human agents. Compressibility is the extent to which an object can be reduced in size by eliminating statistical redundancies. Tamariz & Kirby (2015: 172) define it as "a property of information inversely related to Kolmogorov, or algorithmic, complexity. For a compressible system, it is possible to write a description whose length (in bits of information) is only a fraction of the length of the system itself. The smaller the fraction, the more compressible---and therefore simpler---the system" [Footnote JW: The inverse of this would be the longest binary computer program]. As such, compressibility is a good proxy for visual complexity since visually simpler items or sets of items can be generated by means of fewer rules. Thus it could be argued that a perceived reduction in iconicity across a transmission chain is a symptom of an overall reduction in visual complexity (and compressibility), a possibility already hinted at by Garrod et al. [PK: check this] It should be borne in mind that compression refers not only to pressures that act on individual items within a set, but as a pressure influencing the complete set of items (Enfield 2014). Thus, the compression of a natural script involves both changes to individual graphemes as well as systemic changes within the whole circumscribed set of graphemes that comprise the writing system.