Rationality and Evidence
Among the signs of rationality in one's thinking, the ability
to properly weigh evidence is perhaps one of the most important.
But how good are we at deciding what constitutes evidence and
how do we determine how convincing each piece of it should be?
We may not be as rational as we like to think we are at assessing
To begin with, there are those who are too easily led to believe
what they want and who take almost anything as valid evidence
for that belief. On the other hand, some research shows that
most people are too conservative in assigning weight to a piece
of evidence. In general it is found that we remain less convinced
than the evidence warrants. Or, to put it another way, humans
may be more skeptical than they should be.
Apart from the question of the strength of the evidence, recent
studies also look at how the evidence is presented and what that
does in terms of our weighing of it. Nicole Jardine says in Scientific
...A recent study by Jennifer Whitman and Todd Woodward
found that when pieces of evidence are doled out one at a time,
instead of being shown all at once, people conclude that the
evidence is stronger.
So if a lawyer were to make her case all at once, it would
be less effective than the usual drawn-out process in which pieces
of evidence are introduced one at a time. Or, if you learn three
things which indicate there is global warming, you will be more
convinced if they are presented one at a time rather than all
at the same time. The article continued;
Researchers Whitman and Woodward recently demonstrated
this effect in a controlled laboratory setting. In their study,
people looked at a display on a computer screen. At the bottom
of the screen was a little pond connected to two big lakes. The
pond contained three fish say, two white ones and one
black one. Then the two lakes big lakes were filled with different
proportions of white, black, and yellow fish. People looked at
the lakes and the pond, and used a sliding scale to judge the
probability that the fish in the pond came from Lake 1 or Lake
2. Sometimes there was strong evidence, or a high probability,
that the fish were from Lake 1 (Lake 1 had mostly white fish,
and some black fish, like the pond). Sometimes it was weak evidence,
or a low probability, that fish were from Lake 1. Also, sometimes
the fish in the lakes were added in sequence: all the white fish
appeared, then the black ones, then the yellow ones. Other times,
all the fish were added all at once. When the fish were added
one at a time, people perceived the evidence to be stronger.
Keep in mind that the judgment of participants should have
been the same if the evidence was of the same strength each way
it was presented--at least if it was a process of pure rationality
going on. But just by presenting evidence differently, we get
differing conclusions, even in a laboratory setting. This is
one more example of how imperfect we can be in our thought processes.
It should not be taken as a lack of intelligence (humans have
been to the moon, after all), but it reminds us to be just a
little less certain about all of our beliefs, and a little more
open to ideas or possibilities that contradict our current fallible
thinking. With the possible (and partial) exception of the field
of mathematics, rationality cannot be perfect in humans.