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Can siblings have babies?

The simple answer is no, siblings cannot have babies together. Siblings share genetic material that is too closely related, which increases the chances of birth defects and health complications if they were to reproduce. There are very good biological reasons why incest is taboo across human societies.

What are the genetic risks of siblings having children?

Siblings share about 50% of their DNA. If siblings have a child together, there is a significantly higher risk that the child will inherit recessive genetic mutations from both parents. This increases the chances of genetic diseases and birth defects.

Some of the potential risks include:

  • Higher infant mortality rate
  • Birth defects like cleft palate, club foot, heart problems
  • Mental disabilities and lower IQ
  • Higher risk of rare genetic/chromosomal disorders
  • Weakened immune system

The closer the biological relationship between parents, the higher the risk. The risks are lower for second-degree relatives like cousins, but still elevated compared to the general population.

Why is inbreeding biologically risky?

All people carry some recessive genetic mutations – but they are not expressed when inherited along with a dominant “healthy” gene from the other parent. Inbreeding increases the chances of children inheriting two recessive copies of these mutations, allowing disease traits to manifest.

In a small population with a lot of inbreeding, harmful mutations can become very common. Animal breeders know that inbreeding depression can lower fertility and survival. The risks are elevated for siblings due to how recently they shared the same ancestors.

What are the laws regarding incest?

In most countries, it is illegal for consenting adult siblings to have sex, much less have children. The exceptions are very few. For example:

  • Spain and Portugal allow marriage between uncle/aunt and niece/nephew.
  • The Netherlands and Russia allow marriage between cousins.
  • Some U.S. states like New Jersey and Rhode Island allow adult incest.

However, even where it is legal, social taboos usually discourage incestuous relationships. Many forbid marriage even if sex itself is legal. The laws reflect both genetic concerns, as well as moral views against incest.

Has sibling inbreeding occurred in history?

Yes, there are examples of sibling marriages in history among royalty seeking to preserve a “pure” bloodline or keep wealth and power within the family:

  • Ancient Egyptian royal families sometimes married siblings or half-siblings.
  • Inca emperor Huayna Capac married his sister.
  • King Henry VIII of England’s wives Catherine of Aragon and Anne Boleyn were sisters.
  • Cleopatra’s parents were brother and sister.
  • Some Roman emperors were rumored to have incestuous unions with sisters or nieces.

The risks of inbreeding depression were not well understood. Royal families suffered ill health, and high rates of infant and child mortality as a result. Modern society understands the genetic risks much better.

Are there any examples of sibling couples today?

Consenting adult sibling couples are very rare, but there are a few documented cases:

  • A German brother-sister couple had four children together. The government took custody, but a legal appeal returned the children.
  • A Czech brother-sister couple also fought for the right to continue their relationship.
  • There are extremely rare cases of siblings separated at birth who later unknowingly meet and form a romantic bond.

However, these instances face heavy social stigma and prohibition. Most people understand the health risks today, and refrain from incestuous relationships.

What if siblings cannot have children together?

Some think incest might be permissible if siblings just wish to be in a romantic relationship, but avoid childbearing. However, there are a few issues with this view:

  • Accidental pregnancy could still happen.
  • Not all incest laws or social norms prohibit just reproduction – the incestuous relationship itself makes people uncomfortable.
  • The sibling relationship could be otherwise psychologically unhealthy.
  • Normalizing any form of incest may open the door for abuse.

Overall, there seems to be broad agreement that reproductive issues aside, siblings pairing up violates accepted social and moral norms. Most people find the very notion of sibling sexual intimacy unacceptable.

What about cousins – is that genetically risky?

The risks are much lower for first cousins. Cousins have 12.5% shared DNA, compared to siblings who have about 50% in common genetically speaking. However, some risks remain:

  • 3-4% increased risk of birth defects like heart or skeletal abnormalities
  • 5% increased risk of infant mortality
  • 6% increased risk for rare genetic disorders

The odds are much better compared to sibling incest, but still elevated compared to the general population. 31 U.S. states ban first cousin marriage, while 19 allow it. The social taboo remains, but genetically it is less risky than direct incest.


There are very clear biological reasons siblings should not reproduce. The genetic risks to potential children are considered too high. Additionally, incest breaks strong social norms, and raises concerns of psychological harm even in childless relationships. Though instances exist, sibling incest remains extremely taboo for very understandable reasons in modern society.