Two words: wedding gifts. Host a wedding, and you get to write up a gift registry. And soon, you and your fiance will be positively buried in platters and linens.
Which brings us to a set of islands near Australia -- the Trobriand islands off the coast of New Guinea. Most cultures have crazy elaborate, meticulously-planned wedding rituals, but the Trobriand clans have no real marriage ceremony at all. Couples meet and hook up -- there's never been any taboo against sex before marriage; islanders play erotic games while still children and start having sex as soon as they hit puberty. Couples move in with each other without any formal marriage and then, one day, the couple sits down on the guy's porch for all to see. And that's how the village knows that the relationship is serious.
Despite having three kids, Dave and Amy only got really serious when they sat on their front deck.
The woman's mother comes by and brings a gift -- of plump and tasty cooked yams. They eat these together. The following day, the guy's family arrives with gifts of clothes: skirts, long skirts for his girlfriend. Because if they're going to be a real couple now, it's time to get rid of the wild, wanton wardrobe she wore as a teen.
Then more people from the woman's family arrive, relatives of each of her parents. These folk bring uncooked yams, raw material for many meals to come. Then the man's family comes by with a Bed, Bath & Beyond's worth of cookware, knives, and curios. And then these relatives receive gifts from the bride's relatives -- more yams, uniting all the families in one glorious yam union.
Yams: like gold and jewels, except their value isn't totally made-up.