In the crunchy mothering world, I run into this odd phenomenon. Women get involved in some aspect of mothering that is counter-cultural: extended breastfeeding or natural childbirth for instance - practices that are actually supported by evidence or are merely matters of personal preference. By engaging in this "weird" behavior, they find themselves battling against social norms and doctors' expectations. They find support in a group of like-minded people, and get used to filtering, altering, or outright rejecting advice from uninitiated people, from mothers in law to pediatricians.
Now, it can be perfectly rational to take uninformed advice with a grain of salt. And sadly even experts can be terribly uninformed about, say, breastfeeding. Doctors don't get a lot of training in this area, and often even well-read laypeople will be more up to date on the research than medical professionals.
The problem is, many people seem built not to question everything, but to act more like they're choosing between competing clubs. So many women just seem to see it as a choice between boilerplate belief systems, rather than an investigation into the truth. Either you eat processed factory farmed foods, have an intervention-heavy birth in a hospital, feed your baby formula, vaccinate them, and keep them in a car seat or crib 24/7, OR you eat organic whole foods, have a homebirth, nurse your child until they decide to stop, skip vaccinations in favor of homeopathic remedies, and wear them and cosleep with them.
The choice to reject medical authorities on one subject (which may make sense) tends to lead to rejection of all scientific authority and adoption of crunchy-culture alternatives, even in cases like homeopathy, when it is completely stupid.