We must consider that this expression is always implied in this book and is here to show the boundaries of this knowledge and to remember the caution that must be preserved when it comes to its interpretation.
For example, when we say that being tall and muscular offers a reproductive advantage, we imply “all things being equal”.
In real life, the benefit is not necessarily very obvious. First, maybe that the guy who has this combination will never dare to approach a girl or is impotent, which does not question the principle of the benefit but its realization for the individual.
Then maybe that this genetic combination has other consequences going against the reproduction of its bearer: for example, if, after a disaster, the last girl on earth is traumatized for one reason or another and fear tall men (do not laugh, I know some) she will not consider the handsome giant survivor and will prefer an opportunistic guy like me. In this particular case, it is obviously not an advantage anymore: the principle is challenged and the genes of this man who seemed nevertheless advantaged would disappear.
The human sciences are sciences like others who seek to define general laws based on variables.
The only problem is that these laws are far more numerous in the human sciences than in most disciplines, and most are difficult to quantify or even undiscovered.
This is the opposite approach that was chosen in Evopsy: instead of trying to list the variables, the researcher will consider those he knows and act like if all the others were unchanged.
This is the meaning of “all things being equal”.
Extract from : Evopsy and Seduction