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We investigate how the NIHAO galaxies match the observed star formation mainsequence (SFMS) and what the origin of its scatter is. The NIHAO galaxiesreproduce the SFMS and generally agree with observations, but the slope isabout unity and thus significantly larger than observed values. This is becauseobserved galaxies at large stellar masses, although still being part of theSFMS, are already influenced by quenching. This partial suppression of starformation by AGN feedback leads to lower star formation rates and therefore tolower observed slopes. We confirm that including the effects of AGN in ourgalaxies leads to slopes in agreement with observations. We find the deviationof a galaxy from the SFMS is correlated with its $z=0$ dark matter haloconcentration and thus with its halo formation time. This means galaxies with ahigher-than-average star formation rate (SFR) form later and vice versa. Weexplain this apparent correlation with the SFR by re-interpreting galaxies thatlie above the SFMS (higher-than-average SFR) as lying to the left of the SFMS(lower-than-average stellar mass) and vice versa. Thus later forming haloeshave a lower-than-average stellar mass, this is simply because they have hadless-than-average time to form stars, and vice versa. It is thus the nature,i.e. how and when these galaxies form, that sets the path of a galaxy in theSFR versus stellar mass plane.

NIHAO XXVI: Nature versus nurture, the Star Formation Main Sequence and the origin of its scatter