In the U.S., common law marriage, where it is available, is a function of the laws of the state where it is entered into. Common law marriages are recognized in Colorado, the District of Columbia, Iowa, Kansas, Montana, Rhode Island, Texas, for purposes of the U.S. military, in New Hampshire (for probate purposes only), in Utah (when recognized by a court), and possibly Oklahoma (the state of the law there is unclear).
The usual test, drawn from common law, is (1) an understanding of the parties that they are married, (2) holding themselves out to the public as married, and (3) (sometimes) either cohabitation or consummation.
Cohabitation or Consummation
The modern trend is to focus on the first two elements and to disregard the third element as Colorado has done recently, although, in practice, almost all common law marriage cases are cases in which there has been consummation or cohabitation or both. Texas also does not have the third requirement. The District of Columbia, Rhode Island and Kansas also lack a cohabitation requirement.
Iowa, Montana, and Utah require cohabitation. New Hampshire requires cohabitation for three years.
The original requirement at common law was consummation (i.e. sex between spouses in the married couple). This was a requirement for formal marriages as well, where failure to consummate the marriage within a reasonable time was grounds for annulment (and remains so in some jurisdictions).
The notion of cohabitation in lieu of consummation was an innovation, at least originally, in the nature of a conclusive presumption of consummation that avoided the necessity of testimony about sex in open court, which judges found distasteful when it was not necessary.
In states have have traditionally had a cohabitation requirement, its continued vitality is sometimes in doubt - the issue is often unclear as it was prior to 2021 in Colorado. I have seen attorney-general's opinions in more than one state with common law marriage, for example, that list the first two elements and mention the third element with a question mark as an element that it is unclear whether it applies or not - in part drawing on the fact that consummation is also no longer required in most cases for a valid formal marriage.
No particular duration of cohabitation is required in any of the trust common law marriage jurisdictions in the U.S. except New Hampshire which gives common law marriages recognition only in probate cases (although there are some jurisdictions that recognize rights of cohabitants similar to common law marriage on a temporary basis in a de facto relationship).
Typically, a couple that has signed a lease together or lived together or had sex when while seeing themselves as a married couple (the first element), would be considered to meet the requirement of the third-element in almost all common law marriage states.
For example, if a couple had a ceremonial marriage but forgot to get a marriage license, enjoyed a wedding night where they had sex, and then the wife ended up being deployed with the military before they had moved in together, the requirements of common law marriage would almost always be considered to be met.
The same sex marriage rule has reminded courts that they really don't want to be discussing the question of what counts as sex for common law marriage and hearing testimony about that if they don't have to do so, and since they write the rules, they don't.
Holding Out As Married And Agreement To Marry
Now in the answer linked above it is suggested that a mere declaration
by one party (in a formal written document, specifically a will) that
a marriage exists, without evidence of any agreement by the other
party, or any time living together, would in Colorado in the present
day, create a marriage. This surprised me.
A declaration by one party in a will could probably meet the holding out requirement. It would also require evidence of mutual agreement by the other spouse and capacity to marry. It would generally not require evidence of any time living together. But reputation in the community, while relevant for proving the holding out requirement, isn't a strict requirement as many people simply don't have much of a community and government agencies and official documents that are witnessed are forms of holding out.
If a couple has sent mixed signals, sometimes holding themselves out as married (which some cohabiting unmarried couples do because people assume and they don't bother to correct them every single time), the issue is a question of fact for the finder of fact (usually a judge).
Typically there is a multi-item list of factors that are considered especially relevant in evaluating this legal test, not all of which need to be present.
The "holding out" requirement is typically the core issue, but it has subtle differences in application from state to state.
Capacity To Marry
The couple must also have the capacity to marry without the permission of a third-party (i.e. neither can be currently married, they must usually both be at least 18 years of age, they can't be too closely related to each other). Historically, the capacity to marry rule also barred same sex marriages, although marriages that would otherwise be valid as common law marriages but for the fact that they were same sex marriages have been given retroactive effect in some states including Colorado.
This history of this requirement is somewhat complicated, however.
Colorado has held, both in the cases of same sex marriage and previous cases involving interracial marriages that were void prior to Loving v. Virginia, that a simple void ab initio analysis is not sufficient and that capacity to marry is evaluated at the time an issue is before the court and not when the marriage is entered into by the parties.
Colorado articulated its public policy related to recognition of common law marriage by statute in response to In re Marriage of JMH and Rouse, 143 P.3d 1116 (Colo. App. June 15, 2006) which held that common law marriages in Colorado are not subject to the age restrictions applicable to marriage license marriages in Colorado, only to the limitations of common law, concluding that a marriage between a fifteen year old girl and an older man was valid, even though it would have required a judge's permission, in addition to parental permission from both parents, for them to get a valid marriage license, and that the age of consent for a common law marriage was fourteen for a boy and twelve for a girl.
Similarly, some states that don't themselves allow first cousins to marry will still recognize a marriage between first cousins entered into elsewhere which was valid where it was entered into at the time.
The Case of Colorado
In Colorado, recent caselaw has updated the tests to reflect the constitutional requirement that same sex marriage be legal, in light of Obergefell v. Hodges, 576 U.S. 644, 674–75 (2015), and changing social practices in the cases of In re Marriage of Hogsett & Neale, 2021 CO 1 (January 11, 2021), In re Estate of Yudkin, 2021 CO 2 (January 11, 2021), and In re Marriage of LaFleur & Pyfer, 2021 CO 3 (January 11, 2021).
The Colorado Bar Association summarizes the holding of the cases in a summary of the lead case, Hodges, which states:
The Supreme Court revisited the test for proving a common law marriage
that the Court articulated over three decades ago in People v.
Lucero, 747 P.2d 660 (Colo. 1987). Because many of the indicia of
marriage identified in Lucero have become less reliable,
particularly in light of the recognition of same-sex marriage and
other social and legal changes, the Court refined the test and held
that a common law marriage may be established by the mutual consent
or agreement of the couple to enter the legal and social institution
of marriage, followed by conduct manifesting that mutual agreement.
The core inquiry is whether the parties intended to enter a marital
relationship, that is, to share a life together as spouses in a
committed, intimate relationship of mutual support and obligation.
Once a common law marriage is established, it comes with the same rights and responsibilities as an ordinary marriage, and may not be terminated except by death or divorce.
Application To Opposite Sex Couples
As a practical matter, there are several fact patterns in which common law marriage cases arise between opposite sex couples, some of which are applied by analogy to same sex couples in common law marriage states that have considered the issue.
The most straight forward is that a common law marriage arises when a ceremonial marriage is conducted, without the benefit of a marriage license, or without full compliance with marriage license related laws (e.g. an unrecognized officiant, or a failure to return the license once issues to the appropriate clerk's office).
Another fairly common fact pattern is when a marriage is defective for want of capacity to marriage due to age at the time that the parties have a ceremonial marriage or start to otherwise hold themselves out as husband and wife and consider themselves married, but continue the relationship after the age disability is cured by continuing to act as married.
A third fairly common fact pattern is for a couple to cohabit, to represent themselves as married for purposes such as health insurance benefits, or taxes, or a birth certificate for their child, and for them to come to be known among friends and family as a married couple.
But, a mutual understanding between the members of the couple that they are married, and a single holding out representation to the general public that is not contradicted by other conduct of the members of the couple, could be sufficient.
For example, there is a case out there (I can't find the citation at the moment) involving a couple from a state without common law marriage, that stayed in a motel in a state that had common law marriage and stated that they were husband and wife in the guest register and to the proprietor of the motel, and this was held to be sufficient to establish a common law marriage.
Interstate and Intertemporal Recognition
Other U.S. states must recognize a common law marriage entered into in a U.S. state where common law marriage is valid unless the marriage violates a strong public policy of the recognizing state (e.g. a prohibition on child marriages or a prohibition of bigamy). Likewise, common law marriages that come into being when a state recognizes common law marriage remain valid even if the state abolishes the ability to enter into common law marriage prospectively.
The most common complementary doctrine to common law marriage, usually, but not always arising by statute, and only sometimes found in states that also have common law marriage (Colorado, Montana and Texas have both), is putative marriage. This is available in many states and has been applied by the U.S. Social Security Administration.
The putative marriage doctrine allows someone who believes in good faith that they are married to receive in equity the benefit of marriage, even if they are not legally married. But, the ability to enjoy these benefits ceases to continue to accrue when the person seeking to benefit from the doctrine learns that they are not legally married.
So, for example, suppose that a woman marries a man who already knows that he is married, but she does not know this fact. Until she learns that he is married, she has all of the legal rights of a legal wife, and he has all of the legal responsibilities to her of a legal husband. But, he does not have a right to an equitable division in divorce of property titled in her name or to alimony from her. Once she learns of his prior marriage, however, she ceases to have a right to an equitable division of property he acquires after she learns that she isn't legally married, and any alimony award made to her under the putative marriage doctrine would be an amount she would have been entitled to if she divorced at the moment she learned the her marriage was void.
The putative marriage doctrine can also come into play when a couple has a defective ceremonial marriage and believed that they had a valid marriage license but it was actually defective or not obtained for some reason, until they learn otherwise.
Both common law marriage and the putative spouse doctrine operate basically as curative statutes that protect a less affluent spouse who has lived in good faith as a spouse by providing the economic protections of marriage to someone who hasn't complied with the usual bureaucratic requirements.
Presumption Of Death or Divorce
The existence of a marriage certificate gives rise to a presumption that all prior marriages of both parties to the marriage were terminated by death or divorce at the time of the marriage. This can be rebutted, but proving a negative is sometimes difficult, particularly if the parties to the former marriage that is presumed to have been terminated cannot be found or are no longer living.
De Facto Partnerships
Australia certain kinds of legal effect to certain marriage-like relationships call de facto partnerships under the federal Family Law Act 1975.
Marriage Under Traditional Customs
The Commonwealth of the Northern Mariana Islands does not recognise common law marriage but might recognise customary marriage. In Santos v. Commonwealth (2017), Petitioner argued that common law marriage was sufficiently similar to Carolinian customary marriage that it should be recognised as the same. The Court ruled that, while CNMI statute provides that (English) common law provides the rule of decision in the absence of statutory law or customary law to the contrary, Petitioner did not argue that her marriage was a marriage under customary law but a marriage under common law; thus, whereas a validly contracted marriage under Carolinian or Chamorro customary law might be held a valid marriage, a common law marriage could not be.
Marriage under tribal law is also distinct from state marriage law. Many Native American tribes permit common law marriage or its historic tribal equivalent. For example, the Navajo Nation permits common law marriage and also allows its citizens to marry through tribal ceremonial processes and traditional processes. See Antoinette Sedillo Lopez, "Evolving Indigenous Law: Navajo Marriage—Cultural Traditions and Modern Challenges", 17(2) Arizona Journal of International and Comparative Law 283–307 (2000).