The rule that makes an employer liable for harm its employee causes while doing the job - the reason the trucking company, the hospital or the delivery firm is the defendant, and the reason "independent contractor" is fought over so hard.
Vicarious liability holds one person responsible for another's wrong without any fault of their own. Its main form is respondeat superior: an employer is liable for the negligence of an employee committed within the scope of employment. The employee remains personally liable too, but the employer is the party with insurance and assets, which is why an injury claim against a driver, a nurse, a security guard or a technician is usually a claim against the company they work for. The employer's own carelessness is irrelevant; what matters is the relationship and whether the act was within the job.
"Scope of employment" is where these cases are decided. Conduct is generally within scope if it is the kind the employee was hired to do, occurs within the authorised time and place, and is motivated at least in part by a purpose to serve the employer. A delivery driver's crash on the route is within scope; the same driver's crash on a personal errand across town is usually not, and an ordinary commute is excluded under the "going and coming" rule in most states. Intentional wrongs - an assault by an employee - are harder: some states hold the employer liable where the job created the risk or the act was a foreseeable outgrowth of it, and others require that the act was meant to serve the employer. Sexual misconduct by an employee is treated as outside scope in most states, leaving the injured person to a direct claim for negligent hiring, retention or supervision, which requires proof of the employer's own fault.
An employer is generally not vicariously liable for the negligence of an independent contractor, which is why the classification is contested in injury cases as fiercely as in wage cases. The exceptions are substantial: non-delegable duties (a hospital's duty to emergency patients, a landowner's duty to keep premises safe), inherently dangerous work, and apparent agency, where the injured person reasonably believed the contractor was the company's employee because the company presented it that way. Parents are not vicariously liable for their children's torts at common law, though many states impose limited liability by statute, and vehicle owners are liable in some states for a permitted driver's negligence.
The first thing a lawyer does in an injury case is identify every party who may be vicariously liable, because the individual who caused the harm is rarely the one who can pay, and the deadline to add a defendant is the same as the deadline to sue. The second is to pin down the relationship - employee or contractor, on duty or off, whose vehicle, whose premises - from records the company holds and will not volunteer. If the company says the person was an independent contractor, that is the start of the argument, not the end of it.
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