@Anonymous_
You wrote: "The items of income listed in the "definitions" (from the instructions) are items of income excluded from net investment income and not items of income excluded from the calculation of the NIIT (big difference)."
There is no difference. Net investment income tax is calculated based upon net investment income.
I think we are getting down into the weeds here. Either something is investment income and therefore subject to net investment income tax or it is not. What this all boils down to is Social security is not considered investment income and is not subject to Net investment income tax. The IRS should make reference to the list of items excluded from net investment income in the instructions to line 13 and they dont. The IRS caught the mistake that turbo tax (and apparently at least one other software program) made. Just a google search shows article after article on the net investment income tax and they all say social security is not investment income, hence not included in net investment income subject to the net investment income tax. I just wanted to bring this to the attention of others including turbo tax.
