The results of the study discussed in this post seem obvious when you think about it, but at least there's some empirical data to back it the findings. A study conducted by Pocket Gems suggests that people whose smartphones (including iPhones) have high-resolution screens are more likely to buy in-app purchases from game apps, according to AllThingsD.
Pocket Gems found that "...the iPhone 5 monetizes nearly four times that of the older-generation iPhone 3GS." Likewise, the study found that those using Android devices with high-resolution screens were "nearly 10 times as likely to make an in-app purchase" vs. their lower-resolution counterparts. Interestingly, the iPad did not follow the trend, and did not show a correlation between screen resolution and in-app buying habits.
That's interesting, but I'm not convinced that screen resolution is the determining variable. For example, those who buy the latest and greatest iPhone might have more discretionary income than those still using the 4-year-old 3GS. I don't know if these factors were taken into account, and I also don't know what the study's sample looked like (number, demographic, etc.).
Still, it's an interesting finding. AllThingsD has some infographics that represent additional information.