Not long ago, one of my colleagues posted a link to a paper
from the American Journal of Clinical
Nutrition published in 2010 that analyzed data from a number of studies
involving almost 350,000 people. The
analysis suggests there is no significant relationship between heart disease
and eating saturated fats. He seemed
excited about this result, presumably because it supported his desire to eat
fatty foods.
It is always nice to discover that something you hoped were
true really is true. But, can your
desire for an answer affect the way you evaluate the evidence?
This question was explored in a clever study by Himanshu
Mishra, Arul Mishra, and Baba Shiv published in the June, 2011 issue of Psychological Science. They examined how people evaluated new
evidence when what they believed to
be true conflicted with what they wanted
to be true.
In this study, participants were people who expected to have
children in the near future. All of them
believed that caring for young children at home was better for the child than
sending them to an outside daycare. Of
these participants, half were people who expected they would send their child
to daycare some day, while the other half were people who expected they would
keep their child at home.
The experiment was conducted in a different session from
when the participants expressed their beliefs about daycare and home care, and
so it was not obvious to participants that this study was intended to be
related to their existing beliefs or plans for the future.
In the experiment, everyone read one study that supported
the conclusion that home care really is better than daycare. The other study supported the conclusion that
daycare is better than home-care. The
methods of the two studies were different.
People were asked to evaluate the studies for whether the methods were
valid and whether the studies were convincing.
Not surprisingly, the people who believed that home care is
better and planned to care for their children at home believed that studies
demonstrating that home care is best were more convincing than those
demonstrating that daycare is best.
Those who planned to care for their children using daycare
showed the opposite pattern. Even though
they originally believed that home care is best, they found the study
demonstrating daycare to be best to be more convincing than the study
demonstrating home care to be best.
In many real-world situations, there is conflicting evidence
from different studies. So, it is
important to make judgments about which evidence is strongest. But, these results suggest that people are
biased to interpret the evidence in ways that are consistent with their
desires. That means that people may ultimately come to believe that the weight
of evidence supports the position that they already wanted to believe was
true. And they will believe this without
recognizing that their own desires influenced the evaluation of the evidence.
