1. That people's capabilities can't changeI've learned that this belief is dangerous from the body of work by Carol Dweck and her colleagues which was built over decades. First, research demonstrated that believing that intelligence can't change, which is called a fixed mindset, makes individuals risk-aversive, undermines their learning, persistence and cooperation with others, among other things. Later research showed negative effects of a fixed mindset with respect to many other characteristics, such as personality, emotions, and beliefs.
We cannot change everything about ourselves but most characteristics appear to be more malleable than you might think. Once we realize that, we can see ourselves as a work in progress and get on a learning curve, which may be enjoyable and rewarding.
2. That evil should be fought with evilThis belief can be seen on a micro and a macro level. For example, a few days ago I read an email-exchange between two teachers. Teacher A asked: "How can we punish students for missing class?" Teacher B replied: "Our role is to help them learn, not to punish them." I sympathize with teacher B. Creating a setting of punishment and rewards in a school setting is not a good idea, as research into self-determination theory shows. On a macro level this belief is shown in Donald trump's rhetoric. He legitimizes bad behavior with pointing at bad behavior of other parties. The belief that evil should be fought with evil, I think, harms the motivation of the other party, harms the relationship with the other party, and harms our own morality.
It is important to keep fighting for progress and this may require standing up to harmful behavior. While this may be hard and lead to an asymmetric war, we should do our utmost to fight this fight without lowering our own moral standards.
3. That, after we die, our souls live on and are rewarded or punished for how we have livedThis is perhaps the ultimate example of irrational punishment and reward thinking. Throughout history, people made to believe this claim, have been vulnerable to manipulation by religious leaders. If I can make you believe that you should behave according to a set of rules (which I claim to have a special understanding of) in order to be rewarded after you've died, or in order to avoid being punished after you have died, I can make you do horrific things. As Voltaire said: "Those who can make you believe absurdities can make you commit atrocities". The most salient present day example of what this belief can lead to is suicide bombing in the name of religion.
In fact, although there are some psychological reasons for being vulnerable to the belief that there is life after death, this idea is irrational to a degree that it even can't be true. It is by definition hard to imagine that our conscious experience will once be gone forever. But it becomes easier once we realize that, before our birth, it wasn't there either. Freeing ourselves from this dangerous belief makes us less vulnerable to manipulative religious leaders and makes it easier to appreciate the only life we have.
4. That business should be seen as disconnected from moralityAgain, Donald Trump is probably a good example of someone holding this belief. When confronted with accusations of immoral behavior, he defends himself by saying that he did those things as a businessman. This mindset argues that not paying taxes, not paying bills, outsourcing jobs, and misleading customers are examples of good businessmanship. It is the mentality of: any money is good money. The degree to which this mindset has spread throughout the corporate world is worrisome. Just one important example of a sector in which this mindset creates great harm is the pharmaceutical industry (see here and here).
Making money does not justify harming people and society. Behaving morally does not preclude making money. Business and morality should always be connected.
5. That all different claims about reality are equally trueWhile the belief that all claims about reality are equally true sounds egalitarian and tolerant ("We all have our own truths"), I think it is extremely dangerous. While there may be many situations in which it is not important who is right or wrong, there are also many situations in which it is very important who is right or wrong. Many policies need to be chosen and many decisions need to be made. Every time there are opposing views about what policy to choose or what decision to make, we need some principle or mechanism which helps us do this. Without a belief in the existence of discoverable truth we are extradited to authoritarianism. This may manifest itself in the form of ideological dogma or religious doctrine. If reason and evidence are not available for determining how true claims are, we can only fall back to power as a mechanism to solve disagreements. The rise of Donald Trump can probably also partly be understood by a disbelief in truth. What is true or not does not seem to matter much to Trump and many of his followers. What, instead, only seems to be important is strength and loyalty.
While we probably will never be able to determine unshakeable truths, we can distinguish claims which are very wrong from claims which are more true (less wrong, if you will). Perhaps the greatest invention of humans is a process of how to do this, called science. This process itself is also by no means perfect, thus still a work in progress. But science as it is, is more reliable than any other process in finding out how true certain claims about reality are. Also, science is not something reserved for a small elite. Even when most of us lack detailed knowledge of the scientific process and principles, we can still use some of its main principles when we evaluate truth claims (see here).