The honest answer to this question is, "it depends." However, far too often we're led to believe, or we'd like to believe, that someone can actually predict what we'll do. Although it's true we're like all other people in some respects, we're also like no other person.
I attended a research talk last week by a colleague who specializes in what we call Cognitive Psychology. He's interested in mental processes or how we think. The surprising thing for me is that he presented research on individual differences using personality traits to understand why not everyone responds the same way in experimental situations. The details, although interesting, are not important. The key message is that he learned in his research that people vary, and he's now seeking to understand what might moderate our cognitive processing.
In the course of answering questions asked during the talk, my colleague noted that we typically see these cognitive differences in personality in experiments, but if a task was very important (perhaps outside of the experiment), then people would probably respond the same way. In sum, he added another layer of complexity. First he spoke about the general way we think. Then he showed how people with different personality traits actually think differently. Finally, he commented on how these individual differences may in fact themselves vary depending on the context or the nature of the task.
I was delighted to hear this, because he was actually saying, "it depends." How we'll think, or what we'll do, depends on many factors. Even in carefully controlled experimental studies, we find differences between people. Controlling for these is one of the strengths of social psychology and one of its biggest follies.
Ok, so this isn't so new. We can predict some of the people some of the time. The thing is, we need to remind ourselves of this when we're busy reading the results of psychological studies, or any research on people for that matter. Even drug research reveals that different people (with different genotypes) react differently to drugs and drug interactions.
A listener of my iProcrastinate podcast reminded me of this important point in an email she wrote to me recently. She explained that she had submitted her doctoral dissertation 3 months in advance of her deadline after having battled with earlier procrastination. She credited the podcast as a big help in this change of work habits.
Most importantly, she reflected on how she had to apply the various strategies that have been identified in research studies that I discussed in the podcast to her own context. She explained how her procrastination was a "moving target," changing as she applied strategies and changing as she moved through the process of her writing.
This is so crucially important, I think. Our lives are temporally extended acts. Each project is extended over time as is the very nature of our "self." Perhaps this is best captured by Heraclitus who is quoted as saying "You could not step twice into the same river; for other waters are ever flowing on to you" and "everything flows, nothing stands still." Similarly, Ken Gergen reflects this temporal aspect of our understanding with a critical stance towards social psychology in particular, explaining that social psychology is social history, at least in part.
The last component to this story is one that I have flirted with briefly at the outset: We're like all other people (something we call "human nature"), like some other people (individual differences) and like no other person (idiographically, just me). This is an oft-used quote by two founding figures in personality psychology to introduce students to different levels of analysis in psychology.
In this regard, you might also say that we are like snowflakes. We share many things in common with all other snowflakes (crystalline structure), some things in common with snowflakes from our cohort or snow event (based on humidity, temperature, etc.), and finally we're like no other snowflake in our unique path from sky to earth.
People, like snowflakes, are affected by macro-level forces as well as micro, and predicting the unique interaction of all of these factors over the temporal span of even a single personal project quickly reveals that it is difficult to predict with any degree of certainty (although hindsight, being "20-20," offers us many post-hoc explanations that provide the illusion of the ability to predict, as economists show us every day).
In this fleeting time we hold dear as our lifetimes, we need to make knowledge, personal, applying principles to contexts and the ever-changing flow around us to make sense, as best we can. Of course, we yearn for that crystal ball, and while we may reject the clairvoyant in the circus tent, we can be just as mislead by science at times (particularly media summaries of scientific findings). Even though the predictions are based on more direct evidence, if we apply broad findings without careful thought about context and potential moderating variables, our predictions will probably be wrong.
In the end, I think we're left with one of our biggest personal intellectual challenges. We're left to grapple with the answer, "it depends." Of course, the flip side of that is what we know as personal freedom, human potential and choice.