Fundamentals of Criminal Law by Adam J. McKee

In simple terms, incest refers to intimate relations between individuals who are closely related by blood, such as siblings or parents and children. Such relationships are generally considered taboo in many cultures and are often prohibited by law due to various moral, social, and biological reasons. The specifics of what constitutes an incestuous relationship might differ depending on cultural, religious, and legal contexts. However, at its core, incest is seen as an act involving family members too closely related for it to be considered appropriate or safe.

Legislative Intent: Why is Incest Criminalized?

Lawmakers throughout history have enacted laws against incest to prevent several harms:

  1. Genetic Risks: Biologically, offspring resulting from incestuous relationships have a higher risk of inheriting genetic disorders.
  2. Psychological Harm: Such relationships might arise from or lead to manipulation, abuse, or exploitation, causing psychological harm.
  3. Social Disruption: Incest disrupts family roles and dynamics, potentially causing familial breakdowns.
  4. Moral Foundations: Many societies view incest as morally wrong, often influenced by religious or cultural beliefs.

Thus, incest laws serve to protect potential offspring, the individuals involved, family structures, and society’s moral fabric.

Classification in Legal Codes

Most legal codes classify incest as a crime against persons, given its intimate nature and the potential harm to individuals involved. The penalties can range from fines to imprisonment, with severity often dependent on the specific familial relationship and whether the act was consensual.

Incest within Broader Criminal Categories

Incest belongs to the category of “crimes against persons”, particularly emphasizing the protection of individuals and the sanctity of familial relations. Unlike crimes against property, which focus on tangible assets, or certain sexual crimes that might not involve closely related persons, incest laws are unique in that they concern both personal and familial boundaries.

Historical Roots of Incest Prohibitions

Incest, given its intimate and familial nature, has been a subject of societal concern since ancient times:

  1. Ancient Codes: The Code of Hammurabi, one of the earliest known sets of written laws (ca. 1754 BCE), hints at incestuous relationships being punishable, reflecting society’s early concerns (Roth, M. T., 1997).
  2. Biblical References: The Bible, particularly the Old Testament, contains prohibitions against incestuous relations, highlighting the moral concerns of ancient civilizations (Leviticus 18:6-18).
  3. English Common Law: Sir William Blackstone’s Commentaries on the Laws of England (1769) touches upon incest, primarily as a moral wrongdoing that disrupts familial harmony (Blackstone, W., 1769). Blackstone emphasized the importance of familial roles and the detriments incest posed to society’s fabric.

As society evolved and legal systems became more sophisticated, the definitions and penalties associated with incest have been refined. However, the central concern remains: the protection of individuals and the preservation of familial structures.

Modern Penal Implications for Incest

Over time, societal perceptions have influenced how incest is treated legally. While earlier societies might have been more lenient or inconsistent in punishing incest, contemporary legal systems in many countries have clear penalties in place, reflective of modern understandings of genetics, psychology, and social wellbeing. Current penalties aim to deter potential offenders, protect potential victims, and uphold the sanctity of family structures.

Incest in the Modern Penal Code (MPC)

Modern Penal Code (MPC) Definition of Incest

According to the Modern Penal Code (§ 230.2), incest is defined as marrying or engaging in sexual intercourse, deviate sexual intercourse, or sexual contact with an ancestor or descendant, a brother or sister, or with a child of a brother or sister, whether the relationship is legitimate or illegitimate. The relationships include relationships of the half as well as the whole blood. The severity of the crime varies depending on the exact relationship and the age of the individuals involved.

The MPC’s definition of incest is comprehensive and inclusive. It acknowledges both “traditional” relationships (such as parent-child) and extended familial ties. While many jurisdictions adopt a definition similar to the MPC, specific interpretations and penalties can vary by state or country.

Defenses under the MPC

The Modern Penal Code, while clear on the definition and classifications of incest, does provide some potential defenses to incestuous acts under certain circumstances. These include:

  1. Lack of Knowledge: If the defendant can prove they were unaware of their familial relationship to the other party, it might serve as a defense (MPC § 2.04(3)).
  2. Duress: If the act was a result of compulsion by threat or use of force, where the person reasonably believes that the threat is imminent and inescapable, it might be considered (MPC § 2.09).
  3. Mental Disease or Defect: The MPC may provide a defense if a person, due to mental disease or defect, lacked substantial capacity to understand the wrongfulness of their actions (MPC § 4.01).

However, successfully asserting one of these defenses requires substantial evidence and is not a guarantee against a conviction.

Elements of Incest as Defined by the MPC

For an act to qualify as incest under the MPC, the following elements need to be present:

  1. Mens Rea (Guilty Mind): Knowledge of the relationship and the intention to engage in prohibited conduct.
  2. Actus Reus (Guilty Act): The act of marrying or engaging in prohibited sexual conduct.
  3. Concurrence: The intent and the act must coincide.
  4. Attendant Circumstances: The parties involved are closely related as described by the MPC (ancestors, descendants, siblings).

Degrees of Consanguinity and Incest Laws

“Consanguinity” is a term derived from Latin, which translates to “shared blood.” In the realm of law, particularly when discussing incest, it refers to the level or degree of blood relationship between individuals. Understanding the degrees of consanguinity is essential as many legal systems use this as a foundational basis to define the boundaries of incestuous relationships.

There are two primary systems to classify these degrees: the civil system and the canon or common law system.

Civil System: Under this system, the degree of consanguinity is determined by counting the number of generations between the individuals. For example, a person and their parent are separated by one generation, so they’re considered to be of the first degree. Similarly, siblings, being descendants of the same parents but of different generations, are also of the first degree. Grandparent and grandchild relationships are of the second degree. The more distant the relationship, the higher the degree of consanguinity.

Canon or Common Law System: This method is a bit more complex. It involves counting up the generations to a common ancestor and then back down to the related individual. So, siblings are considered to be related in the second degree (from one sibling up to their parent, the common ancestor, and then back down to the other sibling).

In the context of incest laws, many jurisdictions have explicit boundaries on the degrees of consanguinity that are prohibited from engaging in marital or sexual relationships. For instance, while most cultures and legal systems universally recognize parent-child and sibling relationships as too closely related, there might be differences when it comes to cousins or more distant relations. Some jurisdictions might prohibit first cousins from marrying, while others might allow it, given that they’re often considered to be of a more distant degree of consanguinity.


Incest involves intimate relationships between closely related family members like siblings or parents and children. Historically, many cultures and legal systems have viewed incest as taboo due to genetic risks to offspring, potential psychological harms, disruptions in family roles, and moral concerns. The Code of Hammurabi, the Bible, and even English Common Law documents like Blackstone’s “Commentaries” have addressed incest, emphasizing the importance of preserving family structures and the well-being of individuals.

Modern legal definitions, like the one in the Modern Penal Code (MPC), detail what relationships qualify as incestuous and outline possible defenses, such as lack of knowledge or mental illness. To be classified as incest in the MPC, certain elements, like knowledge of the relationship and the act itself, must align. While society’s understanding and laws regarding incest have evolved over time, the central intent remains consistent: to protect individuals, uphold family sanctity, and prevent potential harm.


Modification History

File Created:  07/17/2018

Last Modified:  09/28/2023

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This work is licensed under an Open Educational Resource-Quality Master Source (OER-QMS) License.

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