Schools of fish; flocks of birds; tribes of people - on a deep level, there seems to be safety in numbers.
But is there? As the Boston Marathon bombings showed, in today's increasingly populous, globalized and urbanized world, crowds freely assembling are prime targets for terrorists. And then there are accidents, such as 2011's Indiana State Fair stage collapse and the Black Friday Walmart stampede in 2008, that remind us of the dangers of crowds.
Does being in a crowd make you feel safer or less safe? Answer the online ballot below, and watch "20/20: In an Instant" Friday at 10 ET for stories exploring this question.