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This research shows the impact of sex and the makeup of the twin pair on the psycho-linguistic performances (formal and pragmatic) of three-year-old twins. Two language assessments were administered to 56 three-year-old twins: 32 same-sex pairs (16 boys and 16 girls) and 24 different-sex pairs (12 boys and 12 girls) in an individual way. Different-sex girls obtain the best results followed by different-sex boys and same-sex girls, with same-sex boys obtaining the lowest results. Being a girl or part of a different sex pair seems to have a positive impact on psycholinguistic performances in three years-olds. In addition, for a twin (girl or boy), it seems preferable to have a sibling of the opposite sex when it comes to the acquisition of specific language rules.