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I’ve long suspected that this is true, but now there’s a formal academic paper proving it:

Winemakers perceive that consumers demand wine with a stated alcohol content that is different from the actual alcohol content, and winemakers are willing to err in the direction of providing consumers with what they want. What remains to be resolved is why consumers choose to pay winemakers to lie to them.

Essentially, people like to think of themselves as sophisticates who go to art-house movies, even if in reality they’re much more likely to sit slack-jawed in front of some reality TV show. In the case of wine, they like the idea of buying something grown-up, with a relatively modest amount of alcohol; when it comes to drinking what’s inside, however, the more heat the better. So wine labels consistently show lower alcohol content than what’s inside.

This is especially true in the new world. Out of 43,908 tested new world wines, 24,561 under-reported their alcohol content, with the reds averaging 14.1% alcohol while claiming just 13.6%, and the whites averaging 13.5% while claiming to be 13.1%.

Interestingly, the smaller number of wines which either over-reported their alcohol content or got it exactly right all reported pretty much the same levels of alcohol: 13.1% or 13.2% for whites, and 13.6% or 13.7% for reds. On average, it seems, wine will just say that it’s 13% if it’s white and 13.5% if it’s red, but in reality it’s likely to be higher than that.

So wine is hot, and getting hotter. Is this a global warming phenomenon? No:

The coefficient on the heat index is approximately 0.05, suggesting that a one-degree Fahrenheit increase in the average growing season temperature everywhere in the world would cause the average alcohol content of wine to increase by 0.05 percentage points; it would take a whopping 20 degree Fahrenheit increase in the average temperature in the growing season to account for a 1 percentage point increase in the average alcohol content of wine.

Instead, it’s a style thing:

We can expect Old World (European) wines to have about 0.63 percentage points less alcohol than wine produced in the New World (the Americas, Australia, New Zealand, and South Africa)… compared with France, three countries produce somewhat lower-alcohol wine (Canada, New Zealand, and Portugal) while the rest produce higher-alcohol wine, with the effects being most pronounced for Australia (0.55 percentage points higher on average) and the United States (0.85 percentage points higher on average).

In Australia, red wine has increased in alcohol content by a whopping 1.4 percentage points over the past 18 years: it’s pretty much an entirely different drink, now, to what it was as recently as the mid-90s.

And yes, the wineries know exactly what they’re doing.

It is relatively inexpensive to measure the alcohol content of wine reasonably precisely (though some of the devices used may entail larger measurement errors), and it is necessary to do so to be informed enough to comply with tax regulations, at least in the United States. It is also an important element of quality control in winemaking. Consequently, we speculate that commercial wineries for the most part have relatively precise knowledge of the alcohol content of the wines they produce and that the substantial average errors that we observe are not made unconsciously. This speculation is based in part on informal discussions with some winemakers who have admitted that they deliberately chose to understate the alcohol content on a wine label, within the range of error permitted by the law, because they believed that it would be advantageous for marketing the wine to do so. In one instance, we were told specifically that the stated alcohol content was much closer to what consumers would expect to find in a high quality wine of the type in question.

If you want to explore the world of high-quality, low-alcohol wines, then, don’t always trust what you see on the label. And doing so is going to be much harder than it used to be. In 1992, white Bordeaux wines averaged 11.7% alcohol; since then they’ve been getting hotter at a rate of 0.35% per year, putting them at 12.5% today. And in the new world the numbers are much higher: Chilean reds, for instance, averaged 12.3% alcohol in 1992, and have been growing in alcohol at a rate of 0.82% per year since then. That means they’re now at 14.2% and rising.

How much further can this trend go? We’re beginning to see a backlash to high-alcohol wines, but I fear that the backlash is simply manifesting itself in lower numbers on the label, rather than lower alcohol levels in the bottle. Winemakers are convinced they know what consumers want — and I fear they’re entirely correct. And consumers, clearly, love to be lied to. So this is likely to continue for a while yet.