The validity of defendant's business deductions on Schedule C of his 2002 income tax return are irrelevant to plaintiff's claim that most of the "nonemployee compensation" reflected on a 2002 IRS Form 1099-Misc she issued to defendant was fraudulently induced by false promises of marriage. Extrinsic evidence may not be used to impeach credibility on a collateral issue ( Halloran v. Virginia Chems., 41 NY2d 386, 390). Any impeachment value of any false statements that defendant may have made to the IRS are far removed from the issues in the case, and clearly outweighed by the strong policy disfavoring disclosure of tax returns ( see Matthews Indus. Piping Co. v. Mobil Oil Corp., 114 AD2d 772 [1985]).