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 
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:
- Parents have no interest in the lives of their children
- Siblings have no interest in the lives of their fellow siblings
- Children have no interest in the lives of their parents (not in England / Wales / NI though Scottish law recognizes this) 
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. 
Credit default swaps
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.
- Kimball-Stanley A. (2008). Insurance and Credit Default Swaps: Should Like Things Be Treated Alike. CONNECTICUT INSURANCE LAW JOURNAL.
- The Free Dictionary definition of Insurable interest, citing cases at 1 Burr. 489. See 20 Pick. 259; 1 Pet. 163. Accessed June 18, 2009.
- Anthony Steuer, Questions and Answers on Life Insurance: The Life Insurance Toolbook, p. 310 (Anthony Steuer, 2007) ISBN 978158348470 found at Google Books. Accessed June 18, 2009.
- Kevin Oldani, You Bet Your Life: Insurable Interest in Life Insurance, found at Society of Actuaries website, citing Life Assurance Act (England 1774). Accessed June 18, 2009.
- "The Insurable Interest in a Life Insurance Policy". Life Insurance Law. Free Advice. 1995-2009. Retrieved 2009-02-11.
- Griffiths v. Fleming  1 KB 805; M'Farlane v. Royal London Friendly Society (1886) 2 TLR 755
- Griffiths v. Fleming  1 KB 805; Married Women's Property Act 1882, section 11.
- Law Commission Insurance Contract Law 
- Report on Family Law; Cohabitation, s. XVI, Scottish Law Commission.
- HMRC 
- EConned, by Yves Smith, 2010, Palgrave Macmillan, p 243, and entire chapter Heart of Darkness