Public speaking bias is a gray area of public speaking ethics. There is no clear ethics rules as to what you should, could, or would say.
Consider the following two examples to see how the decision is made by some professionals.
A group of health care professionals were unable to get into the hospital and lecture on some life saving medical techniques. What they were presenting would in no way be of profit to them. They would not make any money directly or indirectly. Everything they said would only benefit the hospital if implemented. Yet their offers to lecture were repeatedly turned down.
I came along and offered to give the same lecture and not only did they accept my offer, they even closed circuit televised it to two satellite locations so others would be able to attend. Unlike the other speakers, my license as a professional nurse and my certification in the field gave me the needed credibility.
Within days of giving the lecture, the information shared proved valuable. According to one doctor, it was said the information saved a young woman's life.
The first group was not allowed to speak because it violated the hospital's ethics. The question arises, if it was life saving information, if nobody was profiting, why would they not want to know.
Ethical decisions are not always moral decisions in relation to right and wrong. Some ethical decisions are based on the avoidance of bias. If you have a bias or preconceived opinion that may or may not be based on reason or experience, then the information presented could be be one sided.
In this case the hospital had a policy in place. No presentation are allowed where the presenter has any relationship with the source of the information. They wanted total independence.
We all have our personal viewpoints. We have our opinions. To maintain the purity of open minds and avoid biases requires sharing both the good and informing of any pertinent bad that the audience needs to know. What do they need to know? Consider a real life example.
There is a balance to what is ethical when it comes to disclosure. Almost everyone in the hospital gets IV fluid. Patients in the hospital are required to be warned when procedures have potential complications, especially when these are life threatening.
Although statistically improbable, the IV fluid you get, even if just saline water, could kill you. If you were told this and refused the IV and harm came to you, then the telling you would have proved worse than the very slim risk of death from the IV. Just for the record, none of my patients ever died from receiving IV fluids.
So not every bad thing need be told. However if it is relevant to how most people would make a decision, then it needs to be disclosed.
Ask yourself, if you found out a particular piece of information after the fact, how would you feel? Would you rather have known up front?
Public Speaking Ethics Speechmastery.com: The Public Speaking Bias Website
Another reason to avoid public speaking bias is that it makes you look more credible. In one lecture, again to doctors, the down side of the information being presented was shared. One doctor came up and commended me for sharing this, indicating he would not have believed me because of my bias, except for my disclosing both the good and bad.
Avoid bias in public speaking and you will avoid an ethical dilemma later, having to explain why you did not disclose. It will also make you more credible.