if you refer back to my statement, I did not think it necessary the IRS should have obtained their numbers from the same source.
Its been explained multiple times in this thread that until 2011 the IRS had no access to basis information regarding investment income...only proceeds. That is a critically important thing to acknowledge as it can make the difference between owing 10s of thousands of dollars in a given tax year, to getting back a refund due to a capital loss.
Example: Someone sells a stock worth 100k and they paid 110k for that stock. They did not remove any funds from their brokerage account so they were unaware that it was a taxable event. The IRS (prior to 2011) would receive a 1099-B showing the 100k. If no tax return was filed or a return was filed showing everything but that income, they would receive a letter from the IRS asking for taxes to be paid or for additional taxes to be paid. Assuming for the sake of the example a 25% marginal tax rate they would be asking for 25k plus interest and penalties. If the taxpayer did not know any better or did not get competent advice from a tax professional they would pay or set up a payment plan. In reality however, if the tax were properly reconfigured, the taxpayer had a capital loss of 10k, and would therefore owe no tax, or would be due an additional refund from their original return...while having a 7k loss carryforward to carry forward to the following tax year.
Whether or not someone was their parent's dependent for tax purposes would be irrelevant to that calculation.
In reality, because brokers buy and sell stocks within an account many times within a calendar year, the IRS could have received information showing 500k of stock sale proceeds, when in reality the taxpayer only had 100k in the account. Therefore the IRS could be asking for an amount of tax that is greater than their total asset.
I try to get at the truth. I am not here to try and glorify myself or my opinion. Somewhere between everyone's opinions it usually can be found. I notice that neither you or dave jumped in to provide the matrix they will use since OP was likely used as dependent on mom's returns during this period. I think it foolish to kick up potential tax problems for mom and creating a bigger mess. Though you and Dave specialize in tax, I have been an accountant and run various profitability segments in different corporate areas. I may not be specifically a tax expert, I am however a very experienced generalist in several disciplines and industries. That means I am an expert in sales, marketing, finance and human resources. I specialize in making the money. You specialize in reporting it.
You often make pronouncements that are entirely incorrect, and its only very rarely that you acknowledge that your pronouncements were incorrect no matter how much information is provided to you by those who do know the correct information.