If you have a few extra bucks that you don't need for necessities like rent or loan payments, consider shopping for happiness.

From ancient philosophers to current experts in behavioral economics, people have been pondering the link between money and happiness. Among them is author Gretchen Rubin, who thinks about happiness for a living. She's written several books on happiness, including The Happiness Project and the forthcoming Outer Order, Inner Calm.

She helped think through the question of whether you can use discretionary money to buy happiness. Short answer: Probably not. But you can definitely spend money to increase it. A lifetime happiness shopping list might go like this.

Buy better relationships

The key to happiness is how you deal with other humans. It's a recurring theme. "So if you're spending your money to broaden or deepen relationships, that's a good way," Rubin says. Use discretionary money to attend a college reunion or a friend's destination wedding. A corollary, especially for younger adults: Buy a social life. Young adults often experience an intense period of socializing with friends, searching for life partners and networking for career opportunities-all potential sources of happiness. Maybe increase social bar-and-restaurant spending or pay for a dating app.

Buy experiences

The usual advice is "buy experiences, not things", but that requires a deeper dive. "What I find is often the line between experiences and things is not that clear," Rubin says. A bicycle can provide an experience, and a new camera can preserve one. So buy experiences, especially with other people, but also think about buying material things that allow you to have experiences or enhance them.

Buy solutions

Also known as "throw money at the problem" or "buy back time". "One thing that makes people happier is to feel they have control over their time and they're not doing boring chores," Rubin says. So that could mean paying someone else to do yardwork or using a full-service laundry. It's the balancing act of money versus time. If you have a little extra money-probably because you sold your time to an employer-buy back time by paying for convenience.