At a very early stage in your investigation you are going to need to work out what you need to know. Is it literally just the bald facts of what happened, or does it go further into the why? Is there any particular legal, contractual or policy definition in play which will dictate (or at least inform) the decision you have to make? Is the issue limited to that particular employee or should you be looking into the relative treatment of others too? Are you actually being asked to reach a conclusion on law rather than fact, or upon issues which are of a specialist nature outside your technical knowledge?
Take a relative simple bullying grievance as an example. The employee comes to you and complains that he is being “bullied and harassed” by his manager. There has been shouting, unfairness and micro-management, he says. His contribution in meetings is blanked by the manager and he hates coming into work. This has apparently been brought to a head by a recent adverse comment of some sort, perhaps a warning, a downbeat appraisal or some passing slight in the corridor, and he can put up with it no longer. Quite a handful, so what is your way in?
Clearly you are going to have to find out if the bare facts are true – has there been shouting, does the manager ignore the employee in meetings, was there micro-management, etc? By themselves, however, those facts only take you so far. They do not tell you anything about context or motivation, both key to deciding the appropriate way forwards with the employee’s grievance. In particular, those facts do not tell you whether the manager has behaved that way (i) because of the employee’s race or sex or some other protected characteristic (probably unlawful harassment); (ii) because he loathes the employee and wants him to leave (probably bullying); (iii) because he has been driven to it by the employee’s continued inability to make a contribution in a meeting which is worthy of any acknowledgement (probably a coaching question); or (iv) because he was persecuted at school by the other little boys and now thinks this is how you treat people (probably not someone to invite to dinner).
That distinction may not be apparent to the person on the receiving end, but it makes a very substantial difference to the options available to the employer for addressing that employee’s complaint. Remember that harassment occurs where the purpose or effect of the manager’s behaviour is to cause upset or offence, so there is no need for it to be deliberate as a pre-condition of being harassment. For your purposes that is an important distinction – while deliberate harassment will probably amount to gross misconduct, inadvertent or unthinking behaviours may well not do so even if they are still unlawful.
And then you need to consider how your investigation is going to interpret the word “bullying”. Unlike harassment, this has no legal definition at all, so check your internal policies to find out what test you are supposed to be applying. In default of that (or in addition to it) keep in mind that although pretty much every organisation from Acas to Mumsnet has its own definition of bullying, the consistent thread through almost all of them is a requirement of intention or malice. A manager behaving maliciously to a subordinate is likely to face the proverbial long walk on a short pier, but someone behaving the same way because he knows no better is probably better dealt with by some lesser sanction and/or coaching/counselling. If you face push-back from your employee for not dismissing his alleged bully or harasser, you will need to be able to justify your decision, but without that analysis of the drivers behind his conduct as part of your investigation, you will not be able to do that. And if the employee is contributing to the problem, while that is rarely a complete defence to inappropriate management behaviours, it can certainly be mitigation of them
Therefore your investigation here will need to look not just at the factual behaviours of the manager but also at his mindset. For insight into that, you may have to look at his conduct more broadly. If he is similarly ghastly to everyone else then it becomes much harder to conclude that the conduct relates to the individual complainant, whether through discrimination or ordinary malice. While that in no sense makes the conduct acceptable, it is then reasonable for you to conclude that it is a management style problem rather than targeted harassment or bullying, and that the upset which the employee can reasonably take from it must be correspondingly limited.
Therefore before you start investigating off in all directions, sit down and map out on paper the avenues you are going to need to explore. That will be a useful guide but it does not bind you. As the matter progresses, you may find that some of those roads are dead-ends and can be abandoned, while others open up into new areas you will need to look at. So long as you can trace your thought-processes as you go along (like O-Level Maths, you can get credit for showing your workings, even if there are deficiencies found elsewhere), departures from your initial plan should not worry you.