In contrast, there are times in Psychology when it seems amazing that we ever feel confident enough to generalise any findings at all, so numerous are the things that can vary between the participants. Even the most tightly controlled laboratory experiments cannot even begin to account for the differences in the personalities, motivations, beliefs, aspirations, intelligence, family backgrounds, or genetic inheritance of their participants. Even in cases of identical twins, natures most perfect controlled experiment, we know that relatively minor changes in environments (such as having slightly different groups of friends in school) can lead to big differences in the individual. The more we find out about epigenetics, where the environment plays a role in gene expression, which in turn affects how we react to the environment and so on, the more complicated we realise that each individual’s life is, and the more hopeless it seems that we can ever fully understand human life in all its complexity (e.g. see here).
What are the consequences of realising that life is an independent measures experiment? I think that there are both optimistic and pessimistic possibilities. Pessimistically, if taken too far this thought leads us into relativism and the worry that we can never learn anything from anyone else, or even that it is impossible to make informed decisions about our own lives. It certainly suggests that studying Psychology is a bit of a waste of time; what’s the point in psychological theories if we’re all too unique to be analysed by them? Personally, however, I’m more on the positive side. Optimistically, realising our ‘independent measures-ness’ means we can celebrate our uniqueness and avoid sloppy over-generalising and the assumption that simple solutions which worked for one person can be applied to everyone (I see self-help books and ‘How I made my millions’ type books in this category. See 'survivorship bias' for more on these sorts of cognitive mistakes.) Also, the fact that there are no easy answers or universal solutions to life’s problems, rather than suggesting relativism, simply makes it even more important that we base all our decisions on the best evidence available to us. No explanation or theory works for everyone all of the time, but there are some ideas which work for more people than others, more of the time than others, and the role of Psychology is to find this out. We may never be able to predict human behaviour with the precision that a physicist can an electron, but using psychological evidence we can make a decent guess. Whether it works in your own little independent measures experiment can’t be guaranteed, but it’s definitely the best place to start.