Unhappy, negative people can poison a whole office. You can’t spot them just by looking, but you can usually spot them quickly by listening to the constant stream of unhappy, negative comments they make. It’s vital to take action quickly to change the behavior — or remove the negative person — especially in a start-up or small business.

It turns out that one bad apple CAN ruin the whole barrel — if that one bad apple is an employee with a negative attitude who poisons the productivity and cohesiveness of others in his or her work group. That’s something I’ve suspected for years, since I had the misfortune to sit near a chronically unhappy co-worker who made everyone around her miserable.

Recently, I had lunch with a young executive I’ve been mentoring, and he mentioned that he wished he had grounds to fire one of his managers.

“But all he does is grouse more than I like, so I really don’t have cause to fire him,” he said.

Oh, yes you do, I said. If he’s spending time grousing, whispering, and complaining when you can see it, then he’s doing it when you don’t see it too. And whether that takes a few minutes a day — or hours every week — the time he spends complaining is time he’s not doing his job.  So take a close look at what’s not getting done, what projects are running behind schedule, and try to re-focus his attention on what he’s supposed to be doing.

Give him a deadline, and if you don’t see measurable improvement as well as a decline in the amount of complaining going on, then fire him. Why? Because a bad attitude is contagious.

Dr. Will Felps, now a senior lecturer at the University of New South Wales in Australia, proved this in an experiment he performed as part of his doctoral studies at the University of Washington. He assigned groups of undergraduates a series of business management tasks, with $100 per person going to the group that performed best. Unknown to the students, Felps seeded some of the groups with a single bad apple — an actor instructed to behave like a bullying jerk, a slacker or a depressive pessimist. The result: The infected groups performed 30% to 40% worse than the others.

Similarly, Stanford University professors Charles A. O’Reilly III and Jeffrey Pfeffer have written about a Men’s Wearhouse store that had a salesman who was a top producer but was bringing co-workers down by stealing other people’s sales. After he was fired, no single individual was able to match his sales numbers, but sales for the store as a whole rose 30%.

At first glance, you can’t always spot one of these toxic personalities. Like the top salesman who ruined morale by mistreating his co-workers, they can be charismatic, talented individuals. What they do have in common, however, is a penchant for negative chatter that can quickly become contagious.

Whether it’s griping, backbiting, resentment or outright depression, this kind of negative chatter can create big problems. The first problem is that it’s a distraction that a start-up or small business just doesn’t need. More importantly, it’s hard to turn around an office once it’s become infected with a bad attitude. Better to identify and get rid of one negative personality than try to re-focus a whole work-group filled with people who’ve caught the bad attitude from its originator.

How to Spot a Negative Employee

Everybody had bad days. But needy, chronically unhappy personalities can disrupt any workgroup — pay attention, and weed out the most toxic employees as quickly as possible.

One of the sure signs that you’re dealing with a negative employee who needs to be replaced is that they’re proud of their behavior. You can’t motivate them to change, because they think they’re the voice of reason — and anyone who is focused on improving things instead of explaining why things are awful is delusional.

When you run into an employee who seems to be constantly complaining about everything, take a closer look.

First, consider that the person’s complaints are valid. If it’s very specific, and related to a single subject, it might be. But truly negative thinkers complain about everything, and will often point out that someone who thinks that things can get better, or a problem can be solved, is stupid and refusing to face reality.

This kind of negative person is argumentative, and will argue about anything and everything. During the hiring process, they might have covered up their negativity, but once they’re on the job, they usually start being negative relatively quickly. Nothing you can say or do is likely to change them, and getting rid of them is almost always the best choice.

Second, look at whether a negative person might be temporarily depressed or needy. We all go through bad situations — the loss of a loved one, an illness, worry over a tragedy that affects people we care about. But if there’s no obvious cause — or the person isn’t getting better and does not seem to be taking any steps towards changing their behavior (therapy, time off, whatever it takes to improve their outlook) — then you may be dealing with someone who is just a needy, unhappy person.

As I said, everyone feels bad sometimes. Experiencing a blue period is totally and completely normal. But your business isn’t in the business of “fixing” people. Set clear expectations focused on work — deadlines, improved interpersonal relationships — and if they are unable to change their behavior over a reasonable (usually short) period of time, replace them.

Last, but hardly least, pay attention to drama queens with an undertone of anger. Yes, everybody gets upset when they are treated unfairly, or something goes wrong. And sometimes when people get upset, they can behave badly or over dramatize a situation. But most of us understand that acting out isn’t the answer, and we channel our anger in appropriate ways most of the time.

Drama queens (of any gender) can get just as upset over a late food truck delivery as a major product disaster. And it can be hard to tell from their rants and sullen behavior whether something truly truly horrific happened when they were on vacation, or they were disappointed in the amenities in their hotel room. A drama queen can get equally riled up about any problem, and create chaos around themselves complaining about it.

If you have an employee who attacks you, co-workers, customers, suppliers or vendors in any way, you may be dealing with a rage-focused drama queen.  Get rid of them if you spot them attacking anyone physically or verbally, talking about co-workers behind their back, sabotaging a co-worker’s project, or doing anything to bully or make someone else’s life miserable. Trust me, you can’t afford to have someone like that on your payroll.

One caveat: don’t try to diagnose an employee or co-worker’s mental state. Steer clear of any language that implies that they may be depressed, mentally ill, or in need of medication or therapy. Focus solely on their observed on-the-job behavior, not any personal issues they may have discussed at work. For example, talk about the business impact and poor performance that results from complaining instead of solving a problem, focus on missed deadlines or poor performance, and stress that appropriate interactions with co-workers, customers, suppliers and other positive behaviors are a basic requirement of any job with your company.

Document every conversation you have with a toxic employee, and put your expectations in writing. Set a clear deadline, and don’t let the deadline pass without following up to see how they are progressing. Chronic complainers also tend to be litigious, so talk to your legal counsel if your small business doesn’t have a clear set of policies and procedures for terminating employees.