American Sign Language and French Sign Language (LSF) are similar because ASL was directly built from LSF in the early 1800s. About 60% of ASL’s vocabulary traces back to the French Sign Language that was brought to America nearly two centuries ago. The connection isn’t a coincidence or a case of parallel evolution. It’s a direct parent-child relationship between the two languages.
How French Sign Language Came to America
The story starts with Thomas Hopkins Gallaudet, a hearing educator from Philadelphia who traveled to Europe in the early 1800s to learn methods for teaching deaf students. While in Great Britain, he met Abbé Sicard, the head of the Institut Royal des Sourds-Muets in Paris, along with two deaf faculty members: Laurent Clerc and Jean Massieu. Sicard invited Gallaudet to Paris to study the school’s approach, which relied on manual communication rather than forcing deaf students to read lips or speak.
Gallaudet was impressed. He spent time learning sign language from Clerc and Massieu, both highly educated graduates of the Paris school. When he was ready to return to America, he persuaded Clerc to come with him. The two men toured New England, raised funds from both private donors and the government, and in 1817 founded what became the American School for the Deaf in Hartford, Connecticut. It was the first permanent school for deaf children in the United States, and it still operates today.
Clerc taught using the sign language he knew: LSF. The deaf students who arrived at Hartford brought their own local signs, many developed in small communities across New England. Over the following decades, LSF blended with these existing American signs and home signs to form what eventually became ASL. But the French foundation was enormous, and it remains visible in the language today.
Why ASL Isn’t Based on British Sign Language
This is the part that surprises most people. The United States and the United Kingdom share a spoken language, so you might assume their sign languages would be related. They aren’t. ASL and British Sign Language (BSL) are mutually unintelligible. A deaf person from London and a deaf person from New York would struggle to understand each other, while that same American signer would find more common ground with a deaf person from Paris.
The reason is purely historical. BSL developed organically within deaf communities across the United Kingdom over centuries. When Gallaudet traveled to Britain, he encountered resistance from educators there who were protective of their teaching methods. The French educators were far more welcoming, which is why Gallaudet ended up in Paris learning LSF instead. That single decision shaped the entire trajectory of sign language in America. Sign languages follow their own lineage, completely independent of whatever spoken language surrounds them.
60% Shared Vocabulary
Linguistic studies estimate that ASL and LSF share about 61% of their basic vocabularies. These shared signs, called cognates, are words that clearly descend from the same origin, even if they’ve shifted somewhat over two centuries. For comparison, that’s a higher overlap than you’d find between many spoken languages that are considered closely related.
Some cognates are straightforward. The ASL sign for the number three, for instance, has been traced directly to 19th-century LSF. Other shared elements are subtler. ASL inherited a grammatical system from LSF for comparing adjectives, similar to how English uses “bigger” and “biggest.” In both ASL and LSF, signers modify adjective signs with specific movements to show comparative and superlative forms. The distinction between the two, a shorter and softer movement for the comparative versus a more forceful movement for the superlative, exists in both languages and can be traced back to LSF grammar recorded as early as the 1850s.
The remaining 39% or so of ASL’s vocabulary comes from other sources: the home signs and community signs that deaf Americans were already using before Gallaudet and Clerc arrived, plus new signs that have been coined over the past two centuries as the language grew and adapted to American life.
How the Two Languages Diverged
Despite their shared roots, ASL and modern LSF are no longer the same language. Two hundred years of separation has pushed them apart in the same way that Latin eventually split into French, Spanish, and Italian. Signs that started out identical have drifted in handshape, location, or movement. New vocabulary has been invented independently in each country. Grammar has evolved along different paths.
ASL also absorbed influences that LSF never encountered. The local sign systems already in use across America left their mark, and the residential school system in the United States created mixing grounds where deaf students from different regions brought their own signs together. Meanwhile, LSF continued evolving within France under its own pressures and influences. Today, a fluent ASL user visiting Paris would recognize some signs and grammatical patterns but would not be able to follow a full LSF conversation without study.
The relationship between ASL and LSF is one of the clearest examples of how sign languages are true languages with their own family trees, histories of migration, and patterns of change over time. The connection exists not because of any similarity between English and French, but because one deaf Frenchman got on a boat in 1817 and brought his language with him.