Well, I've been doing some more research into this matter, and it seems that traders might actually get away with only paying capital gains tax (box III) of 1.2% on trading profits.
On the website rechtspraak.nl, which is the website that publishes Dutch court decisions (which are only in Dutch unfortunately for those who don't speak the language. Don't know if google translate can be of any help with judicial jargon...), take a look at case
AWB 07/2843 (search for the bold case number and you should be able to find it without any difficulty).
In short, this man, who has worked as an investment professional for years, and who should be considered to know a thing or two about financial products, traded in options privately, made a few (big) losses, and tried to deduct these losses from his taxable income, which in the Netherlands one can do, provided that the trading losses (or gains) are income in box I. One of the requirements to be eligible for that is that a profit from this trading activity was objectively and reasonably to be expected. And that's where the judge has ruled that despite this man's extensive experience in the financial world, which he claimed should give him an advantage over others (and therefore a profit from his trading activity should have been expected reasonably and objectively), still was not sufficient. Unless the man had insider knowledge, which he hadn't, this loss was considered part of normal portfolio management (i.e., box III, capital gains tax of 1.2%), because the judge ruled that the man had no information advantage over other experienced/professional traders, and therefore, a profit was not to be expected objectively and reasonably.
In summary, the judge considers trading an activity that people without insider knowledge should not be expected to profit from (they believe in the efficient markets theorem! :clap
. And therefore, any gains are only taxed at 1.2% in box III, and any losses are not deductible from other sources of income. I should think that with this court ruling in hand, one could easily convince the tax man who thinks otherwise, that trading income should only be taxed at 1.2% as box III income. If not, then you'll be taxed significantly more, but, then again, you can also deduct your losses! And precisely for that reason (given the well known statistic that around 90+% of traders are net losers) I believe that the tax and judicial authorities will leave home traders who are able to live off of their profits alone.