Jean Twenge, a professor of psychology at San Diego State University and the author of "Generation Me" said this reflects a general cultural trend toward individualism and focusing on the self, resulting in a greater emphasis on the extrinsic values of money, status and fame.

Millennials are also gun-shy when it comes to investing in the stock market, Dorsey says, causing them to be more conservative investors, or even to manage their own money. Less money in the market also means they have more cash on hand to spend. The first Financial Independence Survey by The PNC Financial Services Group, released in March, found that only 36% are saving for a house, while only 38% are saving for retirement. It says that most 20-somethings put off thinking about funding retirement until they're approaching 30. Only 13% of 20- and 21-year-olds report saving for retirement, compared with nearly 50% of 28- and 29-year-olds.

Boomers, on the other hand, hard hit by the stock market and with homes that have significantly depreciated in value, are becoming more pragmatic -- though it's also a reflection of age. The older you get, the more practical your shopping habits become, says Elizabeth Harris, senior vice president and strategy director at Leo Burnett USA. Her company forecasts that over the next five years, 64% of millennials will spend more on discretionary items, while that number is only 32% for boomers. She said boomers are more practical and driven about saving for the future, while "millennials are optimistic, engaged and connected, so to them it's more of a journey."

Millennial children are influencing their parents' spending habits in one key area: technology. The two generations spend roughly the same amount of money on electronics. Boomers want to tap the knowledge of their children in this area to appear as young and hip as possible, says Dorsey.

Many millennials also have more money for discretionary purchases because they're being supported by their boomer parents. Twenge says many parents are helping to finance their children's lifestyles, purchasing their clothes and technology. "A lot of 25-year-olds are under the fantasy that money grows on trees. They're not balancing the checkbook," she says.

The members of the generation raised by their boomer parents expect a certain standard of living, says Harris. She points out that among the top five spending categories, only one -- car insurance -- is not discretionary; they're conditioned to not consider these items as "extras." Sands says boomers haven't done their children any favors by funding their upscale lifestyle and failing to teach them successful financial habits. Millennials are much more comfortable going into debt, unlike their boomer parents, Twenge says. And that, she says, could carry some "bad long-term consequences."

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