I couldn’t decide on just one more language acquisition study, like I initially wanted to, so I’ll simply give out a few honorable mentions.
Eric Lenneberg and the critical period hypothesis – In 1967 Eric Lennneberg released a widely influential book based on his research popularizing the notion that if language is not learned before an early age – usually estimated at 4 to 6 years – a child’s ability to learn any language becomes greatly compromised, or disappears altogether. Though this research has been advocated for and debated against by linguistic giants such as Noam Chomsky and Steven Pinker, the evidence from Lenneberg and others is flimsy, draws extensively from widely divergent examples of feral children, and is largely theoretical.
Theory of Mind – Theory of Mind describes the ability to infer the mental states of others. D.G. Premack and G. Woodruff initally espoused Theory of Mind in their seminal 1978 paper, “Does the Chimpanzee have a theory of mind?” Research by Wimmer and Perner in 1983 used a famous false belief task to test study participants’ abilities to put themselves in others’ shoes. Simon Baron-Cohen, Alan M. Leslie, and Uta Frith published research in 1985 suggesting that children with autism have deficits in theory of mind. Research in this area has been widespread, divergent, and often theoretically powerful.
Jean Piaget – The study of his own three children formed the basis for much of Piaget’s work. Not strictly language, per se, but his view of language acquisition was extremely influential, while the middle ground belief (in terms of nature versus nurture) of cognition’s intertwining with language is probably closer to the truth than anything else currently out there. Good info exists here and here. This information is especially informative.