As a general rule, a person who is intoxicated will not be a very good judge of how impaired they actually are. They often get it wrong entirely, completely undervaluing their own impairment.
Researchers have studied this by simply asking people to rate their impairment and then giving them a breath test to check. It has demonstrated that there is a large disconnect. Someone who is highly impaired will often think that they are not that intoxicated, which could be what leads to some issues, such as drunk driving arrests. A person may genuinely believe that they are not too impaired to drive when a breath test would tell another story.
The issue with comparison
The first problem is simply that the person is impaired to begin with. If someone is drunk, their judgment is naturally going to be worse than if they were sober. Simply by virtue of having consumed any alcohol at all, they become less accurate at determining how intoxicated they are.
But the biggest issue is how people have a tendency to compare themselves to others around them. Rather than thinking about how they feel when making the decision about their impairment, they think about how they see themselves in comparison to people nearby.
In other words, one person may feel like they are highly intoxicated if they have had a few drinks and they are in a subdued setting. Another person may be at a party or a bar with people who are highly intoxicated, and they will think they are relatively sober. In actuality, a breath test could show that both people have the same BAC.
Addressing legal challenges
If issues like this lead to drunk driving charges or other legal issues, it is crucial to understand exactly what defense options you have.