Old Journal Articles
I'm an exception. In both psychology and genetics, I have sought out journal articles from every era I could find them. I have an interest in medical history, and in getting a picture of the lives of disabled people in times past.
In the case of genetics, it's true that older articles are of limited scientific usefulness. When they describe a child with 'extra material on one of the D-group chromosomes', it's impossible to tell which chromosomes are actually involved. As a result, you can't compare that child's phenotype with others affected by similar chromosome abnormalities. (Whole-chromosome aneuploidies, the easiest chromosome abnormality to detect, have been reliably described back into the 1960s, however.)
But psychology is a totally different matter. Although we have some new neuroscience measurement techniques, most of psychological research uses similar forms of measurement to what people have used since the beginning of the field of psychology - the technologically simple method of observing behavior and eliciting responses to stimuli. Pretty much the only challenge is that our diagnostic categories have shifted, but if you get used to thinking that 1960s psychotic children are today's autistic children, it's not that big a deal.
And there are some real gems among older studies. In the 1970s, I've found two studies into Machiavellianism in children - one specifically testing interpersonal manipulation ability, another looking at parent-child interactions in the development of Machiavellian beliefs and manipulative behavior. Both studies are quite valuable and informative, and raise several follow-up questions that could be researched further.
Furthermore, there is some data we can only get from older journal articles, such as the gender differences in styles of manipulation used by children in the 1970s. Given shifting gender roles, the results will be different if those studies are repeated now. Culture differs across time periods as well as between different regions, and historical cultural changes are much harder to study. Pretty much the only way to study this is to repeat old studies and see if we find new results in a different cohort.
But we need to remember that just because those articles aren't written with the theoretical knowledge of our time period does not make them useless.