Investing Classics: The Peter Lynch MethodInvest in What You Know — Then Do the Homework

"Buy what you know" is the most misread advice in investing. What Lynch actually meant: daily life or your job hands you a lead—you may see a product catch fire months before the analysts do. But a lead is not a buy, and liking a store is never the same as buying its stock. After the lead, you must study earnings, debt, the source of growth, and valuation, and boil the story down until it's "drawable with a crayon." This is "know what you own, and know why you own it."

L'eggs pantyhose: Lynch's wife Carolyn discovered this hot new product displayed at the supermarket checkout, which led to research on its parent, Hanes. Lynch then did competitive and channel work, confirmed the moat, bought heavily, and it became one of Magellan's early big winners. The point isn't "my wife liked it"—it's that he turned a life lead into a piece of solid homework.

In the 2000 dot-com bubble, countless people bought a stock simply because they'd "used the website"—and went to zero. They stopped at the lead and never did the homework. In a 2015 Wall Street Journal interview, Lynch himself clarified: he never said "like it, buy it," but "like it, then research it." Familiarity manufactures a false sense of safety that lets you skip the interrogation of price and financials.

• Can I explain how this company makes money in crayon-simple terms?
• Beyond "I like the product," have I studied its earnings, debt, and source of growth?
• Is my edge a genuine early observation, or just the comfort of familiarity?
• Have I mistaken "I understand the product" for "I understand the stock's valuation"?

The table above is the script. Classification decides everything: from a stalwart you should expect a 30–50% gain and then bank it, not a tenfold; tenbaggers live almost exclusively among fast growers. The most counterintuitive case is the cyclical—earnings peak at the top of the cycle, so its P/E is lowest and looks cheapest exactly at the most dangerous point to buy; a high P/E at the earnings trough may actually be the opportunity. First identify which type you're buying, then talk about valuation and holding period.

Chrysler, 1982: on the brink of bankruptcy, kept alive by government-backed loans—a textbook turnaround. After researching it, Lynch judged that its cash position had improved and its new models had a market, so he made it Magellan's largest position (about 5% of the fund), and over the next few years the stock rose several-fold. But he managed it on the turnaround script—watching debt and cash flow, wary of a relapse—rather than as a stalwart to hold mindlessly. Get the script right, and only then is a big position safe.

The most fatal error is misclassification: buying a cyclical (steel, airlines, autos) at the top of the cycle as if it were a growth stock, seduced by a low P/E; or falling for a turnaround that never turns—many simply go bankrupt and are outright value traps. Classifying isn't slapping on a label; it's choosing the right risk model.

• Can I clearly state which category each of my holdings belongs to?
• Does my return expectation match the stock's type?
• For a cyclical, do I understand that a low P/E often means peak earnings?
• Does this turnaround have a concrete, verifiable path out—or am I just betting on a miracle?

PEG = P/E ÷ earnings growth rate. Lynch's rule of thumb: PEG < 1 is attractive, ≈1 is fair, > 2 is dangerous. It welds growth to price—a stock at P/E 40 growing 40% (PEG 1.0) may be a better deal than one at P/E 15 growing 5% (PEG 3.0). This rejects two errors at once: paying any price for growth (echoing Day 11's Nifty Fifty lesson), and chasing only a low P/E while ignoring exhausted growth (the cigar-butt trap).

The arithmetic makes it vivid: Stock A at P/E 20 growing 20% has PEG 1.0, fairly priced; Stock B at P/E 40 growing 10% has PEG 4.0—the market has already paid forward years of flawless growth. B's cushion is far thinner—if growth merely slips from 10% to 5%, it suffers a double hit to earnings and to the multiple. This is exactly the yardstick Lynch used to screen out the emotion-inflated names among fast growers.

PEG has clear failure zones: it doesn't apply to companies with no earnings (P/E is meaningless), cyclicals (distorted earnings, inverted P/E), or high-flyers whose growth isn't sustainable. The biggest trap is the denominator—the growth rate is an estimate, and linearly extrapolating past high growth is the most dangerous move, because growth reverts powerfully to the mean. However low the PEG, if it rests on an inflated growth assumption, it's an illusion.

• Is the growth rate I'm using sustainable, or just a straight-line extrapolation of past highs?
• Does this company have real earnings (otherwise PEG doesn't apply)?
• If growth comes in 30% below expectations, is the current P/E still reasonable?
• Have I wrongly applied PEG to a cyclical (where distorted earnings will fool me)?

Stock returns are extremely skewed: a few names supply the bulk of a portfolio's gains. Lynch said "you don't need many tenbaggers"—two or three can define the whole result. The most common error is "cutting flowers, watering weeds": banking a winner the moment it's up 40%, while clinging to a loser hoping to break even. Reverse it—as long as the growth thesis isn't broken, let the winner keep compounding; once the thesis breaks, sell regardless of gain or loss. Hence Lynch: "The best stock to buy is often the one you already own."

La Quinta Motor Inns is one of the tenbaggers Lynch describes: he studied the motel chain's unit economics and expansion runway, confirmed it could keep replicating its profit model, then bought heavily—ultimately about a tenfold gain. At its peak Magellan held over a thousand stocks, yet the overall result was driven by a handful of big winners like La Quinta—one position loses at most 100%, but a single tenbagger offsets ten total wipeouts. That is the power of asymmetry.

There's survivorship bias hiding here: La Quinta is remembered; the many "fake growth stocks" that went to zero are forgotten. And "let your winners run" is never "mindlessly cling through any drop"—the difference between a flower and a weed is precisely whether the thesis still holds (echoing the value trap of Day 5). Moreover, Lynch's edge came from the big moves of small caps; the larger the capital and the more it tilts to large caps, the harder it is to replicate—elephants don't run.

• When I sell a winner, is it because the thesis changed, or just "it went up, I want to bank it"?
• The loser I'm clinging to—does the thesis still hold, or am I just betting on breaking even?
• Can I tell a winner's normal pullback from a thesis that's been broken?
• Are the tenbagger opportunities I chase only found in small caps I can't reach?