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Computer modeling is tracing the hidden evolution of sign languages

It’s relatively easy to trace a written linguistic history—there’s generally a lot of written documentation and records to study. Things get trickier, however, when attempting to examine a sign language’s evolution. Most transformations within the currently over 300 known sign languages (or SLs) around the world occurred sans text over generations of learners. Add in the centuries of marginalization experienced by Deaf and hard of hearing communities, and establishing concrete relationships between SLs becomes extremely difficult.

To help correct this long standing issue, researchers recently created a novel computational program capable of analyzing the relationships between various SLs. The result, published today in Science, is a first-of-its-kind large-scale study that greatly expands on linguists’ understanding of sign language development while challenging long held beliefs about its evolution.

“Many people mistakenly think that sign language is shared around the world, but really the world is full of a vibrant tapestry of different sign languages,” Natasha Abner, study lead author and an associate professor of linguistics at the University of Michigan, writes in an email to PopSci.

For their study, Abner and her colleagues first compiled a video dictionary of core, “resilient” vocabulary across 19 modern sign languages, such as American, British, Chinese, French, Japanese, and Spanish, among others. For example, while a sign for “oak tree” may only occur in languages spoken in regions with oak trees, the concept of just a “tree” is much more ubiquitous. Researchers then broke down video demonstrations for the 19 signing variants for “tree” (along with many other words) into basic phonetic parameters, then entered it all into a massive database.

“What we do in the study is look at how the sign languages refer to these commonplace, universal objects in the world and we work backwards to build a history of the language and languages,” Abner says. “This built history helps us understand the histories of the communities in ways that the historical records cannot because they are so limited and sparse.”

The computational analysis program then examined the signed vocabulary glossary, categorizing each entry based on intricate factors like handedness (one- or two-handed signs), handshape, location, and movement.

“This coding system avoids outcomes driven by superficial similarities or differences in two key ways,” reads a portion of the team’s study. “One, possible character values in the coding system range from two distinct values (handedness) to 10 distinct values (handshape), so it is a highly articulated system capable of capturing and tracking fine-grained differences.”

In tracing signed vocabularies’ evolutions, researchers applied phylogenetic analysis typically associated with biologically inherited traits to physically conveyed communications.

“In our study, the ‘genes’ of language are the words that the languages use to describe the world around them,” says Abner. Pursuing this strategy meant that, instead of simply applying existing computational methods to sign language data, Abner’s team used sign languages “as the empirical basis for advancing the computational methods themselves.”

After examining the dataset, the team’s program established two wholly independent European and Asian sign language families alongside family trees for each one, as well as two distinct Asian sign language subfamilies. Some of the findings reinforced the already known, lasting effects of Western colonization, such as the relationship between British, Australian, and New Zealand sign languages at the expense of endangered or extinct indigenous variants. 

Meanwhile, the documented influence of French sign language within the Western European language tree is backed up by France’s help in expanding Deaf education schools during the 18th century. At the same time, the new computational analysis also revealed previously undocumented connections between British Sign Languages and Western European varieties. To back up the program’s claim, Abner’s team referred back to limited historical records, and found them to corroborate these links.

Abner believes these findings, alongside future advances, will allow sign language linguists the ability to study even more languages and Deaf communities.

“We view this as an important component of demonstrating the equity between signed and spoken languages, and the fact that both are rooted in the biological capacity for language that is part of what makes us human,” she tells PopSci.

“If we want to understand our humanity, then we cannot limit ourselves to spoken languages.”

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