Anindya Ghose wrote about crowds and marketing:

• A study found that those on subway trains are five times more likely to buy things after seeing mobile ads.
• If the subway is crowded then the purchase rate goes up another 45%.
• People dislike having their personal space violated. On trains, they respond to it by turning increasingly inward and focusing on their phones.
• As the real world is tuned out people take in more of their mobile screen – including the ads.

Read more on Recode.