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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.
Driving while intoxicated
All U.S. states have driver licensing laws which state that a licensed driver has given their implied consent to a field sobriety test and/or a Breathalyzer or similar manner of determining blood alcohol concentration. These laws have generally been upheld by courts as a valid exercise of the states' police power, against challenges under the Fourth Amendment (as a reasonable search and seizure) and Fifth Amendment (as not violative of the right against self-incrimination). This is largely because in the United States, driving is considered a privilege rather than a right, and the state has a legitimate interest in keeping dangerously intoxicated drivers off the road, to prevent injury, property damage, and loss of life. In most states, however, the police must have reasonable grounds for administering a sobriety test.
In other contexts
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 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.
- Article from Thompson-Gale Legal Encyclopedia, courtesy of Jrank
- California Vehicle Code § 1808.21(c)
- See e.g. R v R  1 AC 599
- Before You Save a Life: What You Need to Know. Rob Brouhard, About.com. Updated May 8, 2007. Accessed October 30, 2008.
- Keeping kidneys, Bulletin of the World Health Organization, Volume 90, Number 10, October 2012, 713-792