[bolu]

You're right that the article should have been more clear - it's looking at retail investors specifically (in your example; Person2), which is the audience that FutureAdvisor focuses on. It's not focused on other actors in the market such as Institutional Investors, Hedge Funds, etc (you could see those other actors as Person1 and Person3, in your example). You are completely right that if you don't look at the market from any individual perspective there is no inflow and outflow, especially if keeping money in cash is still counted as "in the market".

If we say Person2 is the aggregate of all retail investors, then you have money flow. Person2 in your example has a $100 outflow from Equities and a $100 inflow to Tbonds.

The reason we chose to focus on retail investors and look at money flow from their perspective is to focus on a phenomenon that we see among individual investors, that of moving their money around in the market based on perceptions of the near future, and showing that the data shows this doesn't work.