I like the Back to the Future trilogy for many reasons. Along with Jaws II and Die Hard II, Back to the Future II and III are among the very few sequels that are as good as or even better than the original. And unlike Jurassic Park III or The Core, the Back to the Future storyline is also scientifically credible. Once you accept the premise that time travel is possible in a converted DeLorean, everything else in the story logically follows.

One lesson I draw from Back to the Future II is the difficulty of predicting the future. It’s sobering to realize that we are little more than five years away from 2015, the time of the futuristic society depicted in Back to the Future II, yet we don’t have any of the futuristic devices from the movie: flying cars, hoverboard with or without power (or anything else that can defy gravity), fully automated coffee shops or gas stations without human staff, auto-adjusting and auto-drying clothes, self-tying sneakers, instant food hydrators, voice-operated TV, knobless doors with thumbprint readers, etc. In fact, with the sole exceptions of the internet and cell phones, we are not that technologically more advanced (or qualitatively different) than we were in 1985. (Remember, we already had Apple Macintosh in 1984.) We are not that much more technologically advanced than we were in 1955! We in 2009 are much closer technologically to 1955 than we are to the vision of 2015 in Back to the Future II.

Another example of the difficulty of predicting the future is the painting “Moonport” by Jim Powers. This is Powers’s vision, back in 1956, of what life in the 21st century might look like. Notice that we are still so far away from routine commercial passenger travel to the moon (in shorts, no less), yet we’ve already lost two things that Powers believed would undoubtedly be with us in half a century hence: Big tail fins on a car and !

But that’s not why you called. Back to the Future II is very instructive from an evolutionary psychological perspective, because its creators seem to have grasped the nature and operation of one specific evolved psychological mechanism in our brain.

As any aficionado of the Back to the Future trilogy knows, the actor Crispin Glover, who played Michael J. Fox’s father in the original movie, did not reprise his role in the sequels. He was reportedly fired by the producers for asking for too much money to appear in the sequels. So the role of George McFly in the sequels was played by another actor (Jeffrey Weissman).

As you can imagine, every time different actors play the same role, it presents a headache for the producers. Anybody who’s my age or older will recall the fiasco when Dick Sargent replaced Dick York in the role of Darrin Stephens in the TV series Bewitched. In order to solve this problem, the producers of Back to the Future II came up with a solution that (whether they consciously knew it or not) takes advantage of a blind spot in human nature.

Humans have an evolved psychological mechanism called the face recognition module. Because humans are a social species who lived in a small group which had to compete and fight with other groups, humans developed an ability to tell faces apart very quickly and efficiently. Telling “us” from “them” quickly was a matter of life and death. You know what all of your families and friends look like, and you can spot a stranger immediately. Human mind is particularly sensitive to facial features, to a much greater extent than it is to other natural features and geometric shapes. Recognizing and telling human faces apart come very naturally to humans.

This, incidentally, is why the currency of most nations have human faces on it. Because the human brain is more sensitive to slight changes in human faces than in any other object, this makes it easier to detect counterfeit money. If there is one extra wrinkle on Benjamin Franklin’s face, our mind is likely to detect it, by unconsciously noting that something is amiss, whereas similarly minor alteration in any other designs might not catch our .

Yet the face recognition module works only one way – right side up. Throughout human evolutionary history, there were very very few occasions where our ancestors had to tell faces apart upside down, whether it was the perceiver or the target that was upside down. In virtually all cases where our ancestors had to tell faces apart, they only looked at them one way – the right side up, with forehead above eyes, not below them, and the mouth below the nose, not above it. So the face recognition module works only when the face is presented the right away, not if the face is upside down.

If you want to see this for yourself, take the latest issue of People or Us or any other magazine that has lots of pictures of celebrities. Hold the issue upside down, flip through the pages, and try to identify the celebrities’ pictures in the issue as quickly as you can while looking at the pictures upside down. You will notice that it will always take a fraction of a second longer to recognize the faces, even the familiar faces of your favorite celebrities, if they are presented upside down.

What did the producers of Back to the Future II do? On the pretense that “Grandpa’s back gave out again,” the actor Jeffrey Weissman was suspended midair from some floating device (something else that we don’t yet have) upside down. As a result, the audience saw the actor’s face upside down the entire time he was on screen in Back to the Future II. If the audience didn’t know the switch in actors already, the upside-down suspension of the replacement actor makes it more difficult for them to notice that there’s something wrong.

Whether they consciously knew it or not, the producers of Back to the Future II took advantage of a weakness in an evolved psychological mechanism to avoid potential dissatisfaction in the audience when they had to use different actors to play the same role. Yes, they had the same problem with the role of Jennifer Parker as well, since the role, originally played by Claudia Wells, was reprised in the sequels by Elisabeth Shue. But I suppose the producers figured that her role was too central to the plot, and Shue too big an actress, to keep her suspended midair upside down for the entire movie.