Who reports this income is not determined by who "earns the money". When investments are held in a joint account, the investment income should be reported based on the funds contributed to the account by each spouse. If the funds were provided equally by both spouses, then the investment income would be split 50/50. In the case of a mortgage in both names, it would likely be considered a 50/50 split.
Joint accounts cannot be used to achieve an income splitting tax advantage. In other words, you and the other joint owner cannot arbitrarily split the income 50% each, solely on the basis that it is a “joint” account, or to choose some other ratio to report on your respective tax returns each year to optimize your tax savings.
