Implied consent is consent which is not expressly granted by a person, but rather implicitly granted by a person's actions and the facts and circumstances of a particular situation (or in some cases, by a person's silence or inaction). The term is most commonly encountered in the context of United States drunk driving laws.
All U.S. states have driver licensing laws which state that a licensed driver has given their implied consent to a certified breathalyzer or by a blood sample by their choice, or similar manner of determining blood alcohol concentration. Implied consent laws may result in punishment for those who refuse to cooperate with blood alcohol testing after an arrest for suspected impaired driving, including civil consequences such as a driver's license suspension.
In 2016, the Supreme Court of the United States in Birchfield v. North Dakota held that both breath tests and blood tests constitute a search under the Fourth Amendment, concluding that requiring breath tests is constitutional without a search warrant, however, requiring more intrusive blood tests involving piercing the skin is not, as the goal of traffic safety can be obtained by less invasive means. Specifically addressing implied consent laws, the court in the Birchfield opinion stated that while their "prior opinions have referred approvingly to the general concept of implied-consent laws" that "there must be a limit to the consequences to which motorists may be deemed to have consented by virtue of a decision to drive on public roads" and "that motorists could be deemed to have consented to only those conditions that are 'reasonable' in that they have a 'nexus' to the privilege of driving".
Typically, a party has the right to object in court to a line of questioning or at the introduction of a particular piece of evidence. If the party fails to object in a timely fashion, he is deemed to have waived his right to object and cannot raise the objection on appeal. This is a form of implied consent.
In Canada, implied consent has not been a defence for sexual assault since the 1999 Supreme Court of Canada case of R v Ewanchuk, where the court unanimously ruled that consent has to be explicit, instead of merely "implied".
In many common law jurisdictions, a couple who married were deemed to have given "implied consent" to have sex with each other, a doctrine which barred prosecution of a spouse for rape. This doctrine is now considered obsolete in Western countries.
In the United States, if a person is at risk of death or injury but unconscious or otherwise unable to respond, other people including members of the public and paramedics may assume implied consent to touch the person to provide first aid. Many states have Good Samaritan laws that protect persons giving aid from legal liability, but the type of persons (laypeople versus healthcare professionals) and the amount of protection varies.
Some countries have legislation allowing for implied consent for organ donation, asking people to opt out of organ donation instead of opt in, but allow family refusals.