Just to give you a couple of examples of those,
this is from the Baltimore Longitudinal Study.
Where we're looking at memory, in this case it's errors in doing
a paired-associate task which is I give you a word and
you give me the word that goes with it.
And then I give you the pairs and you study them, and
then give you the test over.
And then look at errors, and you can see the errors go up between age 40 and
age 50.
The errors go up between age 55 and 65 in cross-sectional comparisons, that's
where I'm comparing people at age 40 to people in their 50s at the same time.
These as quasi-experimental cross-sectional studies, 55 and
65, and I see this increase in errors pretty systematically.
What happens when I look at longitudinal data?
With longitudinal data you get the same effect, which is interesting.
So here we have a case that birth cohort with this kind of memory task in this
kind of situation doesn't really have much of an effect.
Because I get the same effects with the longitudinal studies, and
these are the same people being measured at 40, and 50, and at 55,
and 65, not different people, and they get pretty much the same effect.
A more recent longitudinal study of memory was just published by Ronnlund and
his colleagues from Sweden in the famous Betula Swedish Study.
And he's looking at episodic memory, which is memory from things in your past, and
semantic memory, which sort of world knowledge, and he replicates what we found
with cross-sectional studies up until sort of late age.
And that is that episodic memory goes down significantly, while for
a large part of the lifespan the semantic memory stays pretty normal, you can
see it doesn't really change that much up until late age when it starts declining.
But semantic memory is maintained better than episodic memory.
So we do see changes in late age with this longitudinal study that I don't see In
cross-sectional research, but for most of the lifespan we simply replicate
what we see with the quasi-experimental developmental cross-sectional studies.