In theory, two married people share everything equally with each other, including their finances.
In practice, it is common for each spouse to maintain a stash of cash separate from the family finances for his or her own use. This does not always spell trouble for the marriage, but the Detroit Free Press describes several instances in which it could cause a problem.
Most experts agree that it is acceptable, perhaps even healthy, for each spouse to maintain some independent funds. However, while spouses do not necessarily have to account to one another for every penny they spend, they should not hide the fact that they have independent savings from one another. If the existence of the account itself is a secret, this could cause a breach of trust.
During premarital counseling, future spouses may have to answer questions about how much they think is acceptable to spend at a time without telling their significant other. There is no one answer that is correct for every couple, but spouses should agree on an amount that each will accept prior to the marriage. If one spouse is routinely spending thousands of dollars at a time without telling the other, that could be a source of marital strife.
A spouse who frequently makes significant purchases in secret may rationalize that what the other spouse does not know cannot hurt him or her. However, if one spouse runs up large debts, it could hurt the other spouse’s credit.
There are also instances when one spouse may put aside money secretly because the marriage is already in trouble. For example, if one spouse is abusive, the other may put aside money secretly to finance leaving him or her.