I've heard that it happens like this: People are constantly shopping around their scripts to studios through agents, and most scripts are passed on. However, sometimes the same passed around script can influence multiple studios to work on something similar, so they produce a movie on the same subject.

I think that's probably what happened with most of your examples. A Bug's Life and Antz probably came from the fact that the technology at the time made it easier to render insects than other talking animals. And Cheaper by the Dozen 2 is a sequel, so if anything influenced it, it was the previous one (which you could also say influenced YM&O).

Finally, ideas and thoughts are viral. To go back to the volcano example above, who's to say that there wasn't a killer National Geographic documentary on Volcanos on years before the movies came out, and both writers (or story pitchers) saw it and were inspired?

I've always thought that there's a follower mentality. In your examples, the story lines are straightforward and the direction and production are as well. When you get a movie that really takes people by surprise, (say, Pulp Fiction), you get a couple years of cheap ripoffs. Or studios see how well LoTR did and that inspires them to do Lion, Witch & the Wardrobe. The funny thing is that out of your examples, 2 of them are remakes - Cheaper by the Dozen is an old book (I had to read it in 7th grade) and it's a sequel to boot, Bad News Bears was a series of movies that came out during the 70's. Not only are they imitating, they're imitating retread material.

i kinda think there's both fine and coarse grained forces at work with things like this... scripts do get shopped around, and probably have creative influences; then competition amongst hollywood execs probably create arms races to get the works out sooner, resulting in almost simultaneous launches... I think I remember reading about something similar with The Incredibles and the Fantastic Four (haven't seen F4, but apparently, it's very similar in scope, theme, character, and plot to the incredibles...). Antz and Bugs Life? Shark Tale and Finding Nemo? studio catch-up may be involved, but im thinking that technology may have soemthing to do with these coincidences as well... the ability to render convincing blades-of-grass perspective stuff, the ability to render under-water scenes well.. i think the movies followed the tech here.

There are some larger, more subtle examples though... in a 3-4 year period, we saw "Memento", "Eternal Sunshine", "50 first dates", and (yes, this ones a stretch), "Finding Nemo"... all of these movies have the loss of memory as a cenral theme. Might this have something to do with our culture? What does the loss of memory mean to americans these days?

I think it's definitely a common theme to be a copycat. I just watched Sahara last night, and you could probably add Sahara (2005) vs. National Treasure (2004) to your list.

Troy v Alexander (2004)...same deal.

It's a business strategy IMO. It's identical to the one retailers use. Target builds a store in town...and is immediately followed by Wal-Mart or vice versa. Home depot sprouts up...shortly followed by Lowes.

I think the idea is that the actual products sold are largely indistinguishable from eachother. The result is, sales are based on brand recognition and product marketing...instead of the actual value and quality of the product.

The epitome of (...)

I've heard script writers refer to this as "spontaneous generation;" the idea that more than one writer can and will come up with the same idea, at the same time, and write a script about it. They credit it to the fact that people aren't all that different and given the same mass media stimuli we might all come up with the same script idea. This is why almost all studios require writers to have an agent represent them, otherwise producers have to deal with the annoyance of people claiming that ideas were stolen from them, despite the fact that the characters, plot, and story arcs might be completely different. For example, besides anthropromorphic insects, Bug's Life isn't anything like Antz. Anthropromorphic animated characters aren't anything new.