  We show that the dynamics responsible for the variation of the Yukawa
couplings of the Standard Model fermions generically leads to a very strong
first-order electroweak phase transition, assuming that the Yukawa couplings
are large and of order 1 before the electroweak phase transition and reach
their present value afterwards. There are good motivations to consider that the
flavour structure could emerge during electroweak symmetry breaking, for
example if the Froggatt-Nielsen field dynamics were linked to the Higgs field.
In this paper, we do not need to assume any particular theory of flavour and
show in a model-independent way how the nature of the electroweak phase
transition is completely changed when the Standard Model Yukawas vary at the
same time as the Higgs is acquiring its vacuum expectation value. The thermal
contribution of the fermions creates a barrier between the symmetric and broken
phase minima of the effective potential, leading to a first-order phase
transition. This offers new routes for generating the baryon asymmetry at the
electroweak scale, strongly tied to flavour models.
