Bosses with "Jekyll and Hyde" personalities cause their staff the highest levels of anxiety, research has found.
Managers with mood swings result in more tension in the office than those who are nasty all the time, it showed.
The study by the University of Exeter found not being able to predict how a boss would act had a detrimental effect on the productivity of workers.
Researchers carried out four surveys in three companies, two in the UK and one in India, involving 320 members of staff.
They found a poor but consistent relationship with managers was better for workers than one influenced by mood swings.
The impact was worse when colleagues did not have a supportive relationship with their co-workers.
In the study, researchers measured how ambivalent staff felt about their manager - the extent to which they had both positive and negative views about them.
Dr Allan Lee, of the university's Business School, said: "The focus is usually on trying to work out if relationships between staff and bosses are good or bad, but they can sometimes be both, and it is important to measure that.
"Bosses reward and punish their workers, and this has an impact on self-esteem.
"If their staff have to adopt different roles at different times because they have a manager who can be both nasty and nice they view him or her in an ambivalent way.
"It is very negative for an employee to be ambivalent about their boss. Having a clear attitude towards them is much better for their performance.
"If your boss is both pleasant and unpleasant to be around it is hard to know what they think about you, and you can't predict how they will act.
"This makes it hard to trust them. This creates negative emotions and makes staff feel anxious, causing poor performance at work."
The study also found that employees could compensate for an ambivalent relationship with their boss if they had a good relationship with other colleagues.
Researchers asked 60 teams of staff in the three companies what they thought of their bosses and how they performed in different tasks.
The staff who took part in the study worked in a service centre, providing telephone-based customer service support or outsourced human resources services and other tasks.
The study is published in the Journal of Management.