I ask this question in economic terms. Luxury goods are defined as goods that increase as a share of a person’s (or country’s) budget as income increases; necessity goods are defined as goods that decrease as a share of a person’s (or country’s) budget as income increases. There is actually mixed evidence on the topic with some studies saying luxury (Kleiman, 1974; Leu, 1986; Newhouse, 1977) and others saying necessity (Baltagi & Moscone, 2010; Di Matteo, 2003; Farag et al., 2012,; Parkin, McGuire, & Yule, 1987).
