This is true for pretty much everyone - don't go and count yourself as the exception. The more intelligent you are, the more refined your reasoning, but there's evidence to show that intelligence will not lower the bias. Counterarguments from others as intelligent or more intelligent will. One of the curses of being more intelligent is that if you hold a biased view, you usually need someone as smart as you to change your mind. The smarter you get, the fewer people there are who can help remove your bias.
Some people are more objective than others, but often only in a limited domain - not in their whole lives.
>However, 20 per cent of justifications were subjective and involved making a reference to one’s cultural identity, personal experience.
The book also touches on this. In my personal experience, fact based reasoning is rarer than this. There are many reasons people believe something. Attempting to discern the Truth is usually in the minority. It is to be expected that all the other reasons will be more prevalent - they simply have more utility than merely gaining knowledge. It shouldn't surprise people that factual reasoning is rare - it has little utility in most spheres of life. Much less than social cohesion and tribalism does.
Consider the issue of intelligence, and its spread across various groups (usually race and gender). It's very common to find a very well educated person insist that everyone is born equally with the same mental/intelligence potential, and differences exist merely in the extent they foster it. When asked for their rationale/evidence, the answer is usually a variant of "I choose to believe it" (usually for ideological or cultural reasons). I'm not referring only to ordinary folks, but also to university academics, etc.
(I'm not saying that they are factually wrong - merely the reasons they believe it are not based on any facts).
>whether they agreed with the scientific consensus on climate change, vaccines, genetically modified (GMO) foods and evolution
Two of those items (vaccines and GMO foods) touch on a strongly cultural force on purity. The book shows that a lot of people value purity (likely a genetic trait). They associate food consumption not just with physical health, but also mental/spiritual health. So they are quite sensitive to "unnatural" or foreign agents going into their bodies.