What is the standard for determining whether a spouse has knowledge of a substantial understatement in an income tax return?

In addition, when we look beyond the language courts have used in omission cases to the function such a standard has served, we see that it represents merely a different way of approaching what is the same inquiry as the one we announce today. See Purcell v. Commissioner, 826 F.2d 470, 474 (6th Cir.1987). That is, in income omission cases, knowledge of the transaction is virtually equivalent to knowledge of the understatement because if a spouse knows of a transaction which generated income that the return does not report, then it is extremely likely that she will know that the return does not report all income (unless she merely lacks knowledge of tax consequences). Thus, the omission cases that have examined whether a spouse had knowledge of the transaction in a sense really have been looking to discern whether she knew or had reason to know of the substantial understatement.