Yahoo Finance ran a piece this week on mid-cap stocks, promising one with promising prospects and, per its headline, two that underwhelm. You can read the original here. I can't vouch for which specific names made either list, so I'll leave the picks to them and stick to what mid-caps are actually good for.

Mid-caps occupy a useful corner of the market. These are companies with market caps roughly between $2 billion and $10 billion, big enough to have real business models and steady revenue, small enough that they haven't already been bid up by every index fund on the planet. I've held a few over the years. One mid-cap I owned got bought out at a nice premium. Another went nowhere for a long stretch before I finally let it go. Your results will differ, and that's mid-cap investing for you: more possible outcomes, in both directions.

The attraction is real. A mid-cap company has usually survived its awkward startup years. It has management that's been tested, a customer base, and ideally some pricing power. But it still has room to double, which most large-caps simply don't. The catch is that the middle of the market is also where a lot of stalled companies live, businesses that got to a certain size and stopped growing. Separating the growers from the plateaued ones is the whole job, and it's the reason articles like this one exist in the first place.

That kind of sorting has value, even when I don't take the conclusions as gospel. Knowing what to skip is half of investing, maybe more. The piece's framing, one promising name set against names that don't measure up, is exactly the discipline that separates a portfolio from a wish list.

If you're going to wade into mid-caps anyway, a few ground rules I've settled on over four decades. First, earnings matter more than stories. A mid-cap with steady cash flow and a boring product will usually beat a mid-cap with a thrilling narrative and no profits. Second, position sizing. These stocks swing more than the giants do, so a single mid-cap shouldn't be more than a few percent of your portfolio. One stock is a bet, not a plan, and that goes double for the smaller names.

Third, and I'll keep saying this until nobody reads me anymore: most people don't need to pick mid-caps at all. A total market index fund already owns all of them, in sensible proportions, rebalanced automatically as companies graduate up or slide down the size ladder. If a particular mid-cap catches your eye and you want a small side position, fine. I do it myself. But the core of your money belongs in the boring stuff.

So read the piece, take notes, and then ask yourself the question that matters: would I still want this stock if it dropped 30 percent next quarter? If the answer is no, you're renting a headline, not investing.

As always, this is my perspective, not professional financial advice. Do your own homework before you buy anything, especially in the middle of the market.