[13][10] Marcus's early work focused on why children produce over-regularizations, such as "breaked" and "goed", as a test case for the nature of mental rules.

Marcus edited The Norton Psychology Reader (2005), including selections by cognitive scientists on modern science of the human mind.

One of his books, The Birth of the Mind (2004), describes from a nativist perspective the ways that genes can influence cognitive development, and aims to reconcile nativism with common anti-nativist arguments advanced by other academics.

[16] In a review, Mameli and Papineau argue that the theory expounded in the book is "more sophisticated than any version of nativism on the market", but that in attempting to rebut anti-nativist arguments, Marcus "ends up reconfiguring the nativist position out of existence", prompting Mameli and Papineau to conclude that the nativist-anti-nativist framing should "be abandoned".

[10] On 29 March 2023, Marcus and other researchers signed an open letter calling for a 6-month moratorium on "the training of AI systems more powerful than GPT-4" until proper safeguards can be implemented,[21][22] primarily citing the short-term risks of "mediocre AI that is unreliable [...] but widely deployed".

[23] In 2024 he rushed into press his latest book urging public action to regulate generative AI.