| Shrnutí: | <p>We use Horizon-AGN, a hydrodynamical cosmological simulation, to explore the role of mergers in the evolution of massive (<em>M</em><sub>*</sub> > 10<sup>10</sup> M<sub>⊙</sub>) galaxies around the epoch of peak cosmic star formation (1 < <em>z</em> < 4). The fraction of massive galaxies in major mergers (mass ratio <em>R</em> < 4: 1) is around 3 per cent, a factor of ∼2.5 lower than minor mergers (4: 1 < <em>R</em> < 10: 1) at these epochs, with no trend with redshift. At <em>z</em> ∼ 1, around a third of massive galaxies have undergone a major merger, while all remaining systems have undergone a minor merger. While almost all major mergers at <em>z</em> > 3 are ‘blue’ (i.e. have significant associated star formation), the proportion of ‘red’ mergers increases rapidly at <em>z</em> < 2, with most merging systems at <em>z</em> ∼ 1.5 producing remnants that are red in rest-frame UV–optical colours. The star formation enhancement during major mergers is mild (∼20–40 per cent) which, together with the low incidence of such events, implies that this process is not a significant driver of early stellar mass growth. Mergers (<em>R</em> < 10: 1) host around a quarter of the total star formation budget in this redshift range, with major mergers hosting around two-thirds of this contribution. Notwithstanding their central importance to the standard Λ cold dark matter paradigm, mergers are minority players in driving star formation at the epochs where the bulk of today's stellar mass was formed.</p>
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