There are two kinds of shacking up (make sure you choose right!)

The happy kind: Prenuptial cohabitation is where you've already got a ring and a wedding date, or at least the shared understanding that marriage — to each other — is in your not-so-distant futures. There is no proof that cohabiting during a finite period hurts your chances of living happily ever after, and the benefits are obvious: half-price rent and cable, sex on demand, and time to see whether he's evolved enough to put the seat down before you say "I do."

The kind that leads to splitsville: Long-term living together with no clear idea of where the relationship is heading is the type to avoid. Why? You move in on an impulse ("My lease is up; can I move in with you?"), and it's comfy. But once your CDs, finances, and families become entangled, breaking up can resemble a mini divorce. As a result, you may spend years in relationship limbo with someone who isn't "The One." Some couples marry out of guilt (aka, "We've been together so long, we might as well") — only to divorce a short time later. The solution? Set a deadline (six months, a year), get engaged — or get moving.

You might not be on the same page. Surprise! Men and women view living together differently. While you might see it as marriage with training wheels, he may find it merely convenient: the comforts of having a "wife" with no intention of building a future. "Typically, the guy doesn't want to commit," says David Popenoe, founder of the National Marriage Project at Rutgers University. "He also has less to lose by delaying," a nod to the ticktock of a biological clock. If your five-year plan includes marriage and kids, discuss before you U-haul.

Cohabitation can make you fat. It turns out his sweet tooth can hurt you. A recent study by the University of Newcastle upon Tyne showed that women tend to put on weight when moving in with a boyfriend, while men become healthier. Due to a phenomenon researchers call "dietary convergence," when your lives start to merge, so does your choice of snack foods. Translation: He eats more veggies; you adopt his Friday-night nacho habit. Rather than pack on the "cohabiting 10," agree to start a healthy eating and exercise plan together.

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