reliable

What sources do they find reliable?

If you're interested in human performance, what you want is something that's reliable and repeatable, and thus you want neurobiology 'cause neurobiology gives you mechanism.

To abbreviate sharply, we can only willingly tolerate being loved if as children the process of loving and being loved felt sufficiently reliable, safe and kind.

So at the level of neurobiology, we get mechanism and we get something that's reliable and repeatable and most likely will work for everyone, or at least has a much better chance of working for everyone.