Why is the alcohol content in British beer so weak?

I think the reason is the strong pub culture in Britain. The idea is you go to a pub, hang out, have some food, and generally spend an afternoon or evening socializing with your mates. For that purpose, a beer that is lower in alcohol is ideal, since you can sip numerous pints over the course of the evening without becoming drunk to a socially unacceptable level. The culture is so strong in fact that there's even a market for low-alcohol "session" beers with 3.2-3.7% alcohol, specifically intended for extending drinking.

It's the points/levels in the duty imposed that have made UK beer weak, from a historic point of view.

Over 4% and you are on to a higher tariff point for the duty paid on it, so brewers aimed at 3.9x% for a long time, although there have been changes, and a lot of brewers find that people are prepared to pay a few pennies extra for a stronger brew these days.

Pub prices were once highly sensitive to duty, but so much of the price of a pint is now determined by the running costs of the pub, rather than duty, that stronger beers are becoming more common.

When I was a lad, Old Peculier was only sold in 6oz bottles, and a lot of pubs would only sell it in halves on draught ...

The explanation I have been given by a mircobrewer is that it is, indeed, due to the duty levied on British beers - it's not so much a purely economic argument, it's as much a matter of principle of not wanting to give the government too much money. However, there are many excellent microbrewers opening in the UK (based on the American model!) serving their local outlets and they tend to brew stronger beers. A good plan would be to always buy local bottled beers (cask and keg travel better) and you may find something really special. There is a newfound excitement in going to the pub if you are a fan of "real" beers - you may find a gem! The microbrewer I was referring to is the owner of The Kernel brewery in London but there are many popping up around the country.

A possible reason beer was originally drunk in quantity was because it is safer than bad water (beer is boiled during production and the alcohol discourages bacteria). I guess if it was 10% that wouldn't scale very well if you needed to get a day's work done too. There are lots of very strong beers produced in the UK and I'd hazard a guess that not all European beers are 10% so perhaps you're not seeing the full picture

I'm sure it's the "duty" as most people said. I had no idea the percentages were this low until recently. The English beers I saw over here (east coast US) are usually the ESBs and have a normal percentage. Most micros over here have 5-6.9% with special styles being over 7% up to 11%. As a home brewer I noticed the style guides for bitters have some of the lowest alcohol percentages. Even the grain content is slightly under (around 5lbs compared to a California pale of 6-8lbs). The last part would be the most popular yeast probably used for this style ferments out at a higher gravity, then expected for the amount of grains, leaving some sweetness and low alcohol. I enjoy the ESB styles and I'm sure I would enjoy the taste of an ordinary bitter.

3.6% is not an uncommon strength for British bitter, and I strongly suspect that historically, even weaker beer was common.

Brewing beer was the primary method of making water safe to drink and store, in Northern Europe, from prehistory right up until the 19th century when municipal water supplies began to be built.

Since people would be drinking it all day, it makes sense that it would be just strong enough to fend off bacteria, but not strong enough that you'd expect to get drunk -- something like the Kvass that's still popular in central and eastern Europe (typically less than 1.2% ABV).

During the Industrial revolution, manual workers such as coal miners and factory workers would emerge from a long shift, dehydrated from sweat and long hours without a drink, and go straight to the pub. The first pint would barely touch the sides, and you can well imagine them drinking three or four before going home to their families -- wanting only to quench their thirst, not to be too drunk to function in the home. This "ordinary bitter" wouldn't be as weak as Kvass, but it would be nowhere near as strong as a typical modern beer. Perhaps 3% ABV.

India Pale Ale, mentioned elsewhere on this page, is characterised as being brewed stronger and hoppier, so that it could survive the long sea journey to India. Yet I do not believe that a beer would need to be as strong as many modern IPAs (often 5% ABV) to have that property. Hence I suspect a 19th century IPA might have been around 4% ABV, compared to much weaker domestic bitters.

There is a heritage of stronger beers in Britain - porters would be 7% ABV or more, but these are not thirst quenchers to be drunk in quantity.

All this raises the question -- why is British beer now stronger than it used to be? Almost all mainstream lagers are 5% ABV, with 4% variants being marketed as a "responsible" variant when you want to go easy. Where there's bitter on tap, there's often a 5% option. I have in my shopping bag a Scottish beer - Brewdog Hardcore IPA, at 9.2% ABV!

I believe this is a response to competition from European lagers. During the 1970s and 1980s, the fashion for lager almost eradicated traditional British ales from pubs. Alongside the negative stereotypes of the real-ale drinker, the weakness of traditional ales, and their typically mild flavour, made it difficult to compete. The resurgence of real ale in the British market was led by strongly hopped beers, 4.5% - 5% ABV. We are only now seeing weaker beers return to the market, as real ales become fashionable again.

As an aside, in my early 20s, my much older manager taught me the "trick" of choosing a weak beer, on work related pub lunches. "You see," he said, "you can have at least three, and still be able to function when you get back to your desk". A wise man.

One issue that doesn't seem to have been addressed is the effect of 2 World Wars. Significant drops in beer strength and pub hour restrictions were all born from a need to conserve grain supplies for the war. This does help explain the strange closing times of pubs in the UK and Ireland.

The info about the taxation seem spot on!

Beer was once an important part of daily diet in Britain and Ireland and drunk in large quantities by both men and women throughout the day when working, for example, in the fields. So it needed to be weak.

25 years ago when I was a barman in the North of England standard bitter and lager were both around 3%, with only premium brands as much as 5%. And there was Mild, which was often only 2%.

Our drinking culture tends to be about drinking lots of pints, so that is slowly changing, so when Brits ancounter stronger lager and wine, we tend to get knocked down by it.

As others have pointed out, beer was historically a thirst quencher/drinking water substitute rather than a recreational beverage, so there would not have been much incentive to make it stronger in terms of ABV. Rather, the opposite was the case: just enough ABV to inhibit bacteria growth (though the microorganisms at play here were not well understood at the time), but not so much as to impair the consumer’s cognitive skills.

The strong beer in Belgium is in part the result of a previous ban on distilling alcohol. With spirits being out of reach, there was an increased demand for strong beer—just short of 10%, close to the limit of what can be produced by means of fermentation. (The exact amount depends on the type of yeast used, and some purpose-bred yeasts can yield a lot more.)

As a Bavarian who likes to try various European beers, I consider ABV levels between 5–6% normal (not only in Bavaria but also elsewhere in Europe). So British beers tend to be slightly below that, while Belgian beers are slightly above.

Finally, flavor and ABV are not necessarily related. When comparing, say, an Irish Stout (4.8%) to a Lithuanian Tradicinis (6%), the former is definitely richer in flavor. (Don’t get me wrong, there are plenty of great beers to try in Lithuania, should you ever get there :-) So don’t just go by ABV, try (or have your guests try) various different kinds of beer—IPA, porter, you-name-it…