The baby has to fit through the hole in the middle of the pelvis. If a women's hips are narrower I would suspect that there is a higher likelihood of her birth canal being narrower meaning less chance of a successful natural birth.
You really have to discount c-sections and such when thinking about 'fertility' since it would stand to reason that a lot of reactions are instinctual.
> I'd like to see the data on the hip-waist ratio indicating higher fertility
You might have trouble with that if you follow my advice and ignore medical 'intervention' like C-sections since in previous years doctors would do a C-section at the drop of a hat (IIRC, the rate used to be 80% whereas now it's around 60%).
Here is another relevant tidbit I found while investigating:
A previously unexplored reason for the increasing section rate is the
evolution of birth weight and maternal pelvis size. Since the advent of
successful Caesarean birth over the last 150 years, mothers with a small
pelvis and babies with a large birth weight have survived and contributed to
these traits increasing in the population. Even without fears of malpractice,
without maternal obesity and diabetes, and without other widely quoted
factors, the C-section rate will continue to rise simply due to slow changes
in population genetics.
Good point about the c-sections, but still, I am not entirely convinced. For one thing, that wouldn't explain waist/hip, or would it? It would then all be hip, and women would get increasingly larger hips? Also, thinking about Asia, I think women are very tiny there, and they still get children. Their babies are probably smaller, too. But making bigger and bigger babies does not seem to be a "goal" of evolution, either. Otherwise a woman's height would be attractive, which I don't think is the case.
Ah, my mistake. I've seen nothing "conclusive" on that, and I'd highly suspect that the majority of it is just through reasoning.
Slim / average (not anorexic, not overweight) is indicative of health, which is indicative of both healthy babies and a more careful lifestyle, thus more likely a better mother (biologically! not implying deeper connection here).
Meanwhile, wider hips imply a wider birth canal. Combine the two, and you get the hourglass figure, and a relatively arbitrary (/ personal) choice to use 0.7 instead of, say, the golden ratio. (or e/pi, or any other ratio)