Previous research has shown that people are egocentrically biased when making judgements that require a self-to-peer comparison – leading to above-/below-average effects and comparative optimism/pessimism. Two experiments examined whether interhemispheric brain connectivity (assessed via strength of handedness) is associated with egocentrism in the comparative judgement process. In Experiment 1, strong handers (SH) and mixed handers (MH) made percentile rank judgements about their abilities in easy and hard domains. In Experiment 2, SH and MH judged their likelihoods of outperforming a co-participant in easy and hard tasks. Both experiments showed that SH were more egocentric than MH and thus showed (a) more above- and below-average effects when estimating their abilities (Experiment 1) and (b) generally larger optimism biases when predicting performances in a competition (Experiment 2). Taken together, these experiments provide evidence that underlying interhemispheric connectivity shapes egocentrism in comparative judgement.