That's to say, to create science, you have to perform the studies using the methods which provide assurance that the results are accurate.
I'm afraid I doubt the Trobriand Islanders conceive a different way to other mammals. Call me conservative. I'd want to see the academic research.
However, I am not denying that belief can have some effects in particular circumstances. Quite the contrary. New medicine is always tested for its effectiveness against the impact of belief, aka the placebo effect. It is only considered effective if the results show the new medicine is more effective than the sugar pills.
But as far as I know, the desire to avoid pregnancy is not especially effective. Ask a teenager.