  We apply our recently proposed "quadratic genetic modification" approach to
generating and testing the effects of alternative mass accretion histories for
a single $\Lambda$CDM halo. The goal of the technique is to construct different
formation histories, varying the overall contribution of mergers to the fixed
final mass. This enables targeted studies of galaxy and dark matter halo
formation's sensitivity to the smoothness of mass accretion. Here, we focus on
two dark matter haloes, each with four different mass accretion histories. We
find that the concentration of both haloes systematically decreases as their
merger history becomes smoother. This causal trend tracks the known correlation
between formation time and concentration parameters in the overall halo
population. At fixed formation time, we further establish that halo
concentrations are sensitive to the order in which mergers happen. This ability
to study an individual halo's response to variations in its history is highly
complementary to traditional methods based on emergent correlations from an
extended halo population.
