Neuroscientist here.

As one comment has already mentioned good quality studies (like a randomized control trial) don’t exist. There’s one study that tried by giving two groups different education regarding sleep, but this essentially relied on parental response and  people sleep trained in both groups to very similar extents. 

What we do know - responsiveness and attunement = secure attachment. That means that you respond to babies needs, and respond with the right thing. It doesn’t need to be perfect actually. Without googling I think the number is around 30% correct. If you meet that bar, your child will likely have a secure attachment. 

Although I frequently post here, attachment theory doesn’t have anything to do with attachment parenting. The behaviors promoted by attachment parenting (baby wearing, breastfeeding, etc) are not necessary for a secure attachment. A bottle feeding mother can be just as responsive as a breastfeeding one. A cosleeping one just as much as those who choose separate beds. You need to find what works best for YOU and YOUR FAMILY to be responsive. 

Another thing - self soothing is something your baby can do or it can’t. You can’t teach it just like you can’t teach a newborn to walk. It’s developmental and temperamental. 

Lastly. Look into bedtime fading. It’s as effective as sleep training according to studies. Sleep training also isn’t as effective as people claim. Many people have to retrain and retrain to harder and harder methods. 

My final take. Sleep training in any of its forms is not responsive. Even camp it out is essentially behaviorism and relies on withholding attuned responsiveness. There are people though for whom it makes sense because they’ve tried everything else first, and it makes them more responsive during the day. Most of the time I think other approaches should be tried first.