Isn't sexy son hypothesis circular?
The process is self-reinforcing but the argument is not circular (no tautology implied).

As soon as some male traits are considered more sexy than others, then there is selection for females to like those traits even more, which causes those traits to raise in frequency, which increases the selection for liking these traits. In other words, in this model, the state where there are no sexy traits in males is an unstable equilibrium.