ARI SHAPIRO, HOST:

Now to the shadowy underworld of online mattress reviews. When you're getting ready to spend hundreds or even thousands of dollars on a new mattress you want to be smart about it, so you start Googling and reading reviews. And writer David Zax says there are some things you need to know about those reviews. He discovered a kind of Wild West of memory foam where people writing about mattresses have cozy business deals with the companies they're reviewing. Zax described this in a piece for Fast Company magazine and joins us now. Welcome.

DAVID ZAX: Thank you for having me.

SHAPIRO: Your discovery started in the spring of 2016 when you met a guy named Kenny who gave you a free mattress. Who is this dude?

ZAX: Kenny's a nice guy. Let me begin by saying that. He's my neighbor. He's a friend of a friend. And I heard that he had mattresses to spare. So I actually just walked over one day with a bottle of wine under my arm and traded him a bottle of wine for a free mattress. And I sort of asked him, what's going on here? I hear you're handing out free mattresses left and right. And it emerged that he reviewed mattresses online and that he actually got paid a commission from companies when he managed to persuade the people reading his reviews to buy a given mattress.

SHAPIRO: We actually have a clip here from one of his YouTube videos. This one's from 2015.

(SOUNDBITE OF ARCHIVED RECORDING)

KENNY KLINE: Hey, I'm Kenny with Slumber Sage. Today I'm reviewing the Leesa mattress. I've been sleeping on the Leesa about three weeks now. It's been a great experience.

SHAPIRO: So he goes on to rate the firmness of the mattress. And right under the review is a link for a coupon, $60 off the mattress. This has more than 60,000 views. What does Kenny get for doing this?

ZAX: So I did interview the CEO of Leesa, David Wolfe. And with his affiliate marketing partners, as they're called, these reviewers who are also paid commissions on sales they generate, he pays $50 per mattress.

SHAPIRO: You report that some of these mattress reviewers are making more than a million dollars a year from mattress companies just for doing these reviews and doing referrals.

ZAX: It's pretty incredible. But when you think about it - so a mattress is a - it's a big-ticket item. It's a thousand bucks on average, right about. And 5 percent of that, which is a standard commission for salespeople, is $50. So that adds up.

SHAPIRO: We're not going to reveal all the twists and turns of your magazine story, including the surprise ending. But ultimately, the moral seems to be that you can't necessarily trust the reviews you read online, at least of mattresses. How true is this of other review sites?

ZAX: Yeah, well, I wanted my story to be a way into understanding this giant industry called affiliate marketing. $4.5 billion were exchanged - changed hands last year in affiliate marketing. Affiliate marketing basically means, you know, often review sites where a product is reviewed. And on a deep level, the reviewers are incentivized by the very companies that they are reviewing.

SHAPIRO: Because on the review site you can click to buy. And if you click to buy on, say, Amazon, Amazon will give you a cut.

ZAX: Exactly. Or there's a tracking code embedded in a link that goes straight to casper.com or leesa.com. There's no obligation to disclose a case where you're reviewing two competitors and one competitor's paying you $250 per mattress and another competitor is paying you $50 for mattress or $0 per mattress. So it's very difficult to know which sites are honest and which might be less so.

SHAPIRO: What advice do you have for people going online to buy things, wondering if reviewers are getting paid to give positive reviews?

ZAX: You know, definitely snoop around on the website. Check out the disclosures page. Be skeptical of language that says, you know, we may earn a small affiliate commission. Sometimes I tend to think that these disclosures that use the word affiliates, it's almost as though that word is sort of almost boring enough that I think it causes consumers to - eyes glaze over. But that means in some cases that - especially if this is a highly trafficked website - that the people behind that website are making an awful lot of money. That doesn't mean that the review is inaccurate. It's just something consumers should be aware of.

SHAPIRO: That's David Zax, contributing writer for Fast Company magazine. His latest story is called "The War To Sell You A Mattress Is An Internet Nightmare." Thanks a lot.

ZAX: Thank you. Transcript provided by NPR, Copyright NPR.