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Insurable interest exists when an insured person derives a financial or other kind of benefit from the continuous existence of the insured object (or in the context of living persons, their continued survival). A person has an insurable interest in something when loss-of or damage-to that thing would cause the person to suffer a financial loss or other kind of loss.
Typically, insurable interest is established by ownership, possession, or direct relationship. For example, people have insurable interests in their own homes and vehicles, but not in their neighbors' homes and vehicles, and certainly not those of strangers.
The "factual expectancy test" and "legal interest test" are the two major concepts of insurable interest.
The development of the concept of insurable interest as a prerequisite for the purchase of insurance distanced the insurance business from gambling, thereby enhancing the industry's reputation and leading to greater acceptance of the insurance industry. The United Kingdom was a leader in that trend by passing legislation that prohibited insurance contracts if no insurable interest could be proven, notably the Life Assurance Act 1774 which renders such contracts illegal, and the Marine Insurance Act 1906, s.4 which renders such contracts void.
People have an insurable interest in their property up to the value of the property, but not more. The principle of indemnity dictates that the insured be compensated for a loss of property, but not for more than what the property was worth. A lender who accepts a house as a mortgage, has an insurable interest on the property used as security, but the insurable interest is not in excess of the value of the loan.
Insurable interest refers to the right of property to be insured. It may also mean the interest of a beneficiary of a life insurance policy to prove need for the proceeds, called the "insurable interest doctrine". Specifically, insurable interest is:
- An interest based upon a reasonable expectation of pecuniary advantage through the continued life, health and bodily safety of another person, and, consequently, loss by reason of their death or disability; or
- A substantial interest engendered by love and affection if closely related by blood or by law.—Society of Actuaries 
The principle of insurable interest on life insurance is that a person or organization can obtain an insurance policy on the life of another person if the person or organization obtaining the insurance values the life of the insured more than the amount of the policy. In this way, insurance can compensate for loss. A company may have an insurable interest in a President/CEO or other employee with special knowledge and skills. A creditor has an insurable interest in the life of a debtor, up to the amount of the loan. A person who is financially dependent on a second person has an insurable interest in the life of that second person.
Legal guidelines have been established in many jurisdictions which establish the kinds of family relationships for which an insurable interest exists. The insurable interest of family members is assumed to be emotional as well as financial. The law allows insurable interest on the presumption that a personal connection makes the family member more valuable alive than dead. Thus, husbands/wives have an insurable interest in their spouse, and children have an insurable interest in their parents (and vice-versa). Brothers/sisters and grandchildren/grandparents are also assumed to have an insurable interest in the lives of those relatives. But cousins, nieces/nephews, aunts/uncles, stepchildren/stepparents and in-laws cannot buy insurance on the lives of others related by these connections.
A person is presumed to have an insurable interest in his or her own life, preferring to be alive and in good health rather than being sick, injured or dead. The unlimited interest extends to the life of spouses (and since 2004 civil partners), even if there is no financial dependency.
UK law does not recognize other classes of so-called 'natural affection' however, thus:
Nor is insurable interest recognized for cohabiting couples. Although many insurers will accept such policies, they could potentially be invalidated because they have not been tested in court. In recent years, there have been moves to pass clear statutory provisions in this regard, which have not yet borne fruit.
In practice these problems are solved by people assigning their policies or placing them in trust with named beneficiaries. If a person obtains an insurance policy on their own life, it is presumed that the person would only name a beneficiary who wants the insured to be alive and healthy. There is no requirement that the beneficiary have a proven insurable interest in the life of the insured when the insured has purchased the insurance.
In 2008, the English and Scottish Law Commissions tentatively proposed some reforms to the existing law, hoping to clarify the complex rules. Their preliminary recommendations included increasing the category of ‘natural affection’ to include dependent children and parents and also cohabitees. Officially this is still under review. 
In eConned, Yves Smith argues that credit default swaps were/are used to take out insurance-like contracts against financial products in which buyers had no insurable interest. This was related to the financial crisis of 2008 because hedge funds and others allegedly helped produce bad subprime mortgages on purpose so that they could buy insurance on them, and then profit when the home buyers failed to make payments.