Early on, he and his colleagues discovered that 2-year-olds display previously unsuspected cognitive capacities, providing alternative explanations for Piagetian demonstrations, and contributed to a shift from the constructivist viewpoint toward biologically grounded theories that required validation with much younger infants.

In Trieste, his group became interested in how the process of statistical, or distributional learning (a non-language-specific mechanism) in infants might interact with their capacity of extracting and generalizing algebraic-like structures from their perceptual input.

Along with his students and collaborators, Jacques also explored adult speech processing, arithmetic abilities, music, social cognition, executive functions in bilingual infants, and human reasoning.

For example, The Proceedings of the National Academy of Sciences (2009) describes the research conducted by Agnes Melinda Kovacs and Jacques Mehler on the cognitive gains in seven-month-old bilingual infants.

In three eye-tracking studies, Kovacs and Mehler found that infants, reared with two languages from birth, display improved cognitive control abilities compared with matched monolinguals.