There’s no shortage of automakers offering gas-electric hybrid versions of their cars—most of which look identical to their gas-powered counterparts. And yet, the decade-old Toyota Prius remains the untouchable hybrid hit. A record 28,711 Prius cars (which, in addition to the original hatchback, now include a subcompact and wagon) were sold in the U.S. in March. Another 1,039 were sold in Canada—a 215 per cent rise from a year earlier. The Prius accounts for nearly half of all hybrids sold in North America. Why? One theory, tested in a paper last year by Ph.D. candidates Steven and Alison Sexton, is that hybrid owners are mostly interested in appearing “green,” and that the futuristic-looking Prius was the best car for the job. They called the effect “conspicuous conservation.” The message for carmakers: even when it comes to the environment, never underestimate the vanity of your customers.