They say opposites attract -- but when it comes to money, that can be a recipe for disaster. According to "Shark Tank" investor Kevin O'Leary, financial misalignment is the leading cause of marital breakdown, far surpassing infidelity.
"The No. 1 reason for breakups in marriage is not cheating -- it's financial pressure," O'Leary told GOBankingRates. "It's one partner outspending the other."
O'Leary, who recently partnered with HelloPrenup, an online prenuptial agreement platform, believes couples must be aligned on one key financial habit: spending behavior.
Check Out: Barbara Corcoran Put $62 Million Into Her 'Shark Tank' Investments -- How Much Did She Make?
Read Next: I'm a Retired Boomer: 6 Bills I Canceled This Year That Were a Waste of Money
Before tying the knot, O'Leary urges couples to have honest conversations about their financial priorities. He recommends discussing questions like:
"These things really, really matter in the first three years of a marriage," he said. "You can see disastrous outcomes if somebody is a spendaholic and the other is not, and they never discussed this or talked about their financial goals in the first place."
Discover More: 3 Best Ways To Prioritize Your Savings Goals
O'Leary emphasizes that money should be treated as a joint responsibility, but that doesn't mean merging everything.
"The first child you have in a marriage is money," he said. "That's the very first child -- but you both have to have separate bank accounts."
His advice? Maintain individual accounts for personal finances, and fund a shared account for mutual expenses. This structure allows couples to work toward common goals while protecting their independence.
"All of that has to be in place," he said. "And a prenup is a perfect way to structure that."
Whether you're newly engaged or already married, aligning on spending habits isn't just smart -- it's essential. O'Leary believes the first 18 months of marriage are critical for syncing up on how you spend, save and support each other financially.
"This is absolutely crucial," he said. "It's the most important thing."
More From GOBankingRates
This article originally appeared on GOBankingRates.com: Kevin O'Leary: The One Money Habit Couples Must Align On To Avoid Divorce