Psychological Aspects Children receiving X- Mas presents

Relative is trying to severely limit Christmas gift giving to the extended family’s kids. Using an anti-materialist, trying no to spoil them, teach them values argument - that I find sanctimonious and obnoxious (though she is just proposing it, not demanding it). I expect that I will get hammered by folks who feel the same way about X-Mas and I don’t say that she doesn’t have a point… Which makes it all the harder for me to argue against it by being all about materialism, spoiling kids and being anti-values if you follow.

Having said all that, My GQ is:
Are there any studies or articles written by reasonable AUTHORITIES that show a positive effect on kids receiving gifts? Anything to hang my argument on besides sounding like a indulgent (and indolent) Santa?

How about Parental Gift Giving at Christmas: An Exploratory Study . This ought to be academic enough for you.

However, my own personal opinion: Gift giving at Christmas often serves several meanings. Societal folkways, personal mores, and simple satisfaction of making someone feel elevated by giving a gift.
Can a child lose the real reason behind Christmas, and giving gifts in the spirit of giving? Yes, of course. Are there ways other than those mentioned by the person you speak of in the OP to instill values in children? Definitely.