It's 'common knowledge' that Mac and Linux gaming are so unimportant that they might as well not exist. Everyone 'knows' that Mac games sell a tiny fraction of the amount that Windows games sell, and that Linux games simply don't exist.
But are these ideas based on reality? Where did these figures come from? Anyone who has ever worked with statistics knows that it's unwise to trust 'common knowledge' without data to back it up. I decided to ask some developers about their sales distribution, and get some real data about Mac and Linux game sales.
Blockbuster game sales
When we talk about the games industry, we usually think of blockbuster games. With such high volume, surely they would be indicative of the state of Mac gaming, right? Aspyr Media publishes Mac ports of big releases like Modern Warfare and Civilization 4, and they reported that their Mac ports sell 3-6% as many copies as the Windows versions. This is well above the common 1% idea, but still not very much. Why is it so low? Do Mac users just not play games?
Do Mac users play games?
Since online Flash games are free and cross-platform, I thought they could help us determine if Mac users play games or not. I asked Kongregate (a popular Flash game site) what operating system the players were using. Kongregate reported that 90% of world-wide players use Windows, 9% use Mac, and 1% use Linux. Since Macs have only a 6% market share world-wide, this is surprising -- it suggests that Mac users may actually be more likely to play games than Windows users!
Indie game sales
So blockbuster games don't find their audience on Mac, but what about independent downloadable games? To find out, I asked a number of independent developers about their sales distributions, including the authors of Machinarium, Gish, World of Goo, Grappling Hook, DROD, and Penumbra. Surprisingly, the average sales distribution was 72% Windows, 22% Mac and 6% Linux!
Is this just true for obscure indie games? What about large-scale casual releases like Plants vs. Zombies or Bejeweled? I asked Popcap games about their sales distribution, and they reported a similar figure: 20-25% of sales are for Mac.
This is a very different picture than the one we got from blockbuster games. 6% compared to 25%? How is this possible? Here is one possible explanation.
Effective market share
According to browser statistics, Apple's worldwide market share is about 6%, and Linux market share is about 1%. This gives us the following market share (users per platform): 93% Windows, 6% Mac, and 1% Linux.
However, what we really want to know is how well a game would sell on each platform -- the effective market share. The NPD survey shows that Mac users make twice as many electronics purchases. It's not much of a stretch to speculate that they also spend twice as much on small entertainment purchases like games. Let's suppose that there are also half as many Mac games as there are Windows games. That means that the average Mac user spends twice as much on half as many games, which means they are four times as likely to buy any given game. I don't have hard data about Linux users, but I'll go out on a limb and say that they tend to be tech enthusiasts, and have to choose from an even smaller selection of games, so let's say their multiplier is 50% higher than Mac users.
If we multiply the initial market share for each platform by their increased spending (4x for Mac and 6x for Linux), and adjust to add up to 100%, we get this effective market share: 75% Windows, 20% Mac, 5% Linux
This is much more in line with the figures we see from indie and casual game sales. While the methodology for my explanation is purely speculative, the resulting graph closely matches the sales data reported by developers. Given these effective market shares, this raises a new question about blockbuster games. Why do they only sell 3-6% of their copies on Mac, instead of 20%?
So why don't the blockbuster games sell?
Indie games usually do not have dedicated advertising campaigns, and sell slowly as word spreads through gaming communities. However, Blockbuster games are sold very differently. They often spend more on launch advertising than on actual development -- for example, Modern Warfare 2 spent $50 million on development, and $150 million on launch advertising. This leads to an initial spike of inflated customer interest in the game, which falls off dramatically over the course of a month or two. Here is a graph of Modern Warfare sales with data from VGChartz:
As you can see, the Mac version was released a year after the Windows version, as customer interest and awareness was reaching an all-time low. I realized that the Mac percentages are so low not because the Mac versions sell poorly, but because the Windows sales are artificially inflated by the marketing campaign. If these games were released simultaneously for both platforms, I would expect the sales distributions to look more like we would expect from our effective market share chart.
The future of cross-platform gaming
It's a good time to be a Mac gamer. Valve recently announced Steam for Mac, and that all their future games will be simultaneously released for Mac and Windows, including Portal 2.  Also, there are a number of other big games coming up that will be released simultaneously for Mac and Windows, including Diablo 3, Starcraft 2, and Rage. Some of the big developers are starting to realize that there are a lot of Mac gamers, and that porting to Mac is not very hard. If you can put in an extra 1% cost and get 22% more revenue -- why not make a Mac version? It would be worth it even if it were only an extra 2% revenue!
However, they haven't yet taken the next step: once you've made a Mac version, indie games show that Linux users represent an additional 6% effective market share. Once you've created a Mac port and divorced your code from DirectX and other Windows dependencies, it's not that difficult to port to Linux. Our most recent promotion, the Humble Indie Bundle, shows even more dramatic statistics for Linux: 52% Windows, 24% Mac, and 24% Linux.
With Overgrowth, we were already planning a simultaneous Mac, Linux, Windows release just out of principle, in order to include everyone. However, it's nice to see that the numbers are on our side as well!