Saturday, September 18, 2010

On sex and singles

So, the post title is clearly designed to pump up pageviews, but those of you who have come here hoping to see photos of me with dollar bills hanging out of my G-string are going to be sadly disappointed. The good news is the money you'll save having your corneas scraped.

This post is actually about the evolution of sex, or "recombination," as the biologists like to call it. The question is, why does sex exist? Or, at a genetic level, why would an organism do something that passes on only half of its genes (by mating with something that donates another half), rather than simply making a genetic copy of itself. This is often referred to as the "two-fold cost of sex." Presumably, there must be an evolutionary benefit to sex that is great enough to overcome this two-fold cost.

As with everything in evolutionary biology, there are an enormous number of theories that have been proposed to explain the evolution of sex, but there are two major arguments. One is that sex allows beneficial mutations that arise on different backgrounds to be recombined onto a single genetic background. This allows adaptive evolution to occur at a faster rate. The other (which is really sort of another side of the same coin) is that sex permits more efficient purging of deleterious mutations.

Let me use an analogy that requires us to take a walk down memory lane. You kids may not know this, but a long time ago, music came on albums, which contained a bunch of songs. The problem with the album system was that most bands would put out one good song, and then fill the rest of their album up with crap. So, to get a collection of good songs, you had to buy a whole bunch of other songs that you didn't actually want. Sure, you could buy the 45, but who did that, seriously?

So, in this analogy, the first theory, the one about beneficial mutations, is like how you would take all of your albums and put the best songs together on a mix tape that you give to a girl you're trying to impress. Yes, back then, this was done non-ironically by people who were not hipsters. She would then listen to the first few songs out of a sense of politeness, make some awkward comment about how knowledgeable you are, and then mysteriously change her phone number.

One of the great things about the advent of mp3s and digital music sales is that it is easier to hide your embarrassing musical taste. It used to be that your friends would always pull out your Night Ranger album and make fun of you. Now you can rock out to Ke$ha and just close your computer when someone knocks on your office door.

Also, and more relevantly, it is easier and more natural now to buy individual songs. So, you don't ever wind up owning a whole pile of non-I'm-Gonna-Be-(500-Miles) Proclaimers songs. Music has undergone a transition to where it is more like our second theory, where recombination permits the elimination (through failure to purchase) of deleterious mu(sic)tations.

I'd write more, but there's a pile of cash on the dresser that I need to count.

Monday, September 6, 2010

On evolution and sequels

So, there are a lot of things in evolution that seem like they are moving in one direction, when actually they are moving the opposite way. Or maybe it's the other way around – I forget. For instance, one of the things that we know is that the vast majority of naturally occurring mutations are deleterious. That is, just like your crotchety old grandfather always said, children are, on average, a little bit worse than their parents (and the music they listen to is A LOT worse). Yet, somehow, evolution is able to maintain a level of function in the face of these deleterious mutations, and even to create new adaptations.

The reason is natural selection. Children will be worse than their parents on average, but there will be variation. Some will be a lot worse, and some only a little worse. Some may even be a bit better. The key is that the better children will, on average, produce more grandchildren than the worse children will (so your nagging mother was also right). It's a bit like walking the wrong way on one of those people-movers at the airport.

Of course, there is also noise in the system. Sometimes a big rock falls on the "fittest" individual in a way that has little to do with that individual's genotype. And sometimes an individual carrying a lot of deleterious mutations starts a polygamous cult and has about a hundred kids. But on average, the filtering effects of selection seem to counterbalance, or even outweigh the effect of those deleterious mutations.

This got me wondering if there was maybe something similar going on with movie sequels. The conventional wisdom in most quarters is the movie sequels suck. Sure, there is the occasional Godfather II, but for every one of those, it seems like there are a hundred films that are closer to Highlander II. So, I did a little study [1], in which I compared three classes of films: movies that got sequels, movies that are sequels, and random movies. Two scores from Rotten Tomatoes were collected for each movie: the "tomatometer" score, which is the percentage of reviews of the movie that were positive, and the user score, which is the average rating (out of 10) by users of the site.

The average scores are:

Movies with sequels: 59.2% positive 5.92 average (coincidence, or Illuminati plot?)
Movies that are sequels: 44.8% positive 5.16 average
Random movies: 45.7% positive 5.21 average

So, what's our conclusion here? Well, it seems like sequels are, on average, pretty darn similar in quality to the random sample of movies. The outlier is the set of movies that get sequels made. So, maybe we think that sequels suck because we tend to mentally compare them with the originals, and, like our high-school sports careers, they fail to live up to expectations. Maybe sequels suck because movies suck, and a sequel is no more or less likely to suck than anything else. Or is there something about sequelness in itself?

We can drill a little deeper by dividing our movies into five quintiles (with ten movies each) based on the tomatometer scores of the originals:

Bottom quintile:
Movies with sequels: 17% positive 3.6 average
Sequels of movies: 15% positive 3.6 average

Second quintile:
Movies with sequels: 45% positive 5.2 average
Sequels of movies: 22% positive 4.0 average

Third quintile:
Movies with sequels: 58% positive 5.9 average
Sequels of movies: 49% positive 5.3 average

Fourth quintile:
Movies with sequels: 82% positive 7.0 average
Sequels of movies: 64% positive 6.1 average

Top quintile:
Movies with sequels: 94% positive 7.9 average
Sequels of movies: 74% positive 6.8 average

What this makes it look like is that there really is something about making a sequel that makes your movie suck more than the original. For the most part, you can expect a 15-20% drop in the number of favorable reviews going from the original to the sequel, even if the sequel was only average to begin with. The one exception is the bottom quintile, where you can expect your sequel to suck just about as much as the original did. This may be a boundary effect, as the average number of positive reviews is bounded at zero. This is the great thing about making "Baby Geniuses 2" is that it is virtually impossible to underperform "Baby Geniuses." On the other hand, with a tomatometer score dropping from 2% to 0%, the baby geniuses somehow managed it.


[1] Not-very-scientific study methodology:

In order to collect a sample of sequels, I went to Rotten Tomatoes, and searched for "2" and "II," discarding anything that was obviously not a sequel, or for which there was no rating information available. This yielded a list of 50 movies, including "2 Fast 2 Furious," but not "Aliens." For each of these, I got the "tomatometer" score and the average user rating for that movie and for the movie of which it was the sequel.

For the random sample, I went to The Movie Insider, and used their list of January-June 2009 releases. I discarded anything that was a foreign film or documentary that had an initial release date prior to 2009. The rationale here was that if a documentary is shown at film festivals in 2007, and then gets a major theatrical release in 2009, this is not a random movie. It is a movie that has already undergone a fairly intense selection process. In the end, this list had 75 movies in it.

The study was not double-blind or vetted by anyone else, and undoubtedly contains errors in both transcription and judgment. However, hopefully it is close enough for analogic use.