Have 10-15% of the general population had at least one incestuous sexual contact when under the age of consent?

Question

I found these numbers in the page Incest at Wikipedia:

Incest between an adult and a person under the age of consent is considered a form of child sexual abuse[70][71] that has been shown to be one of the most extreme forms of childhood abuse; it often results in serious and long-term psychological trauma, especially in the case of parental incest.[72] Its prevalence is difficult to generalize, but research has estimated 10–15% of the general population as having at least one such sexual contact, with less than 2% involving intercourse or attempted intercourse.[73] Among women, research has yielded estimates as high as 20%.[72]"

Reference:
[72] Courtois, Christine A. (1988). Healing the Incest Wound: Adult Survivors in Therapy. W. W. Norton & Company. p. 208. ISBN 978-0-393-31356-7.
[73] Nemeroff, Charles B.; Craighead, W. Edward (2001). The Corsini Encyclopedia of Psychology and Behavioral Science. New York: Wiley. ISBN 978-0-471-24096-9.

But these numbers don't sound right to me. Sadly, I don't have the books to check any other details relevant to the studies or the methodology

Have as many as 10-15% of the general population had at least one sexual contact that is incestuous when they were under the age of consent, as suggested by the Wikipedia quote?

Answer 1

The first reference is to Healing the Incest Wound: Adult Survivors in Therapy, by Christine Courtois.

An online copy of the original book is available to borrow for free.

We can confirm that on page 5, it makes claims consistent with the Wikipedia page:

Since 1978, research efforts to learn more about the characteristics and patterns of child sexual abuse have intensified. Among some of the most striking and the following:

• A very substantial percentage of the female population, possibly as high as 20%, has had an experience of incestuous abuse at some time in their lives, 12% before the age of 14, 16% before the age of 18. Possibly 5% of all women have been abused by their fathers (Russell, 1986).
• Boys are also sexually victimized within the family, but in smaller numbers (Finkelhor, 1984).

Looking in the bibliography, we get fuller references:

• Finkelhor, D. (1984). Child sexual abuse. New theory and research. New York: The Free Press
• Russell, D.E.H. (1986). The secret trauma: Incest in the lives of girls and women. New York: Basic Books

The second book is also available to borrow for free.

Chapter 4 (page 59-) is titled The Prevalence of Incestuous Abuse in Contemporary America. (Note: This is the first time we have seen the statistic limited to the USA - in fact the study was done only in San Francisco.)

The introduction to the chapter makes a special effort to define the terms:

Incestuous includes any kind of exploitive sexual conduct or attempted sexual conduct that occurred between relatives, no matter how distant the relationship, before the victim turned eighteen years old.

It is worth noting that this includes attempted sexual conduct, which may be broader than the original "sexual contact" claim. "No matter how distant the relationship" is rather vague (we are all related, very distantly!) but having the victim (who knows the strength of their own familial connections) make the call about who is in their family might be the best approach.

They surveyed women in San Francisco, sampled by a public opinion research company. The respondents replied to an initial letter, and were then interviewed in person.

They found (p60):

Nineteen percent of the sample of 930 women reported at least one experience of incestuous abuse at some time in their lives.

but went on to clarify that 40 of them were initiated after the age of 18, and so were excluded from their definition for analysis, leaving 16% of women who experience incestuous abuse as a child.

(I note that the author has published an article in 1997 that updated the earlier book, Healing the incest wound: a treatment update with attention to recovered-memory issues, that does not include any prevalence claims.)

In summary:

• I have confirmed that Wikipedia fairly quoted the Courtois reference.
• Courtois, in turn, slightly changed the claim from Russell (A sample of 19% was turned into as "up to 20%". The limit to the USA/San Francisco was omitted. Attempted sexual conduct was equated to abuse. The broad definition of family wasn't covered.)
• Russell got the figure from her team's own survey, taken in San Francisco.
• I haven't confirmed Courtois claims about boys.
• I haven't confirmed the claims cited to the Corsini Encyclopaedia, including the 10-15% of the population claim.
• However, if 16% of underage girls are affected, and women make about 50% of the American population, then we have a lower bound (i.e. in the hypothetical extreme case that no boys are affected) of 8% of the overall prevalence. This makes the actual 10-15% overall prevalence claim seem more plausible.
• I've made no attempt to see if Russell's survey was replicated or reviewed. I focussed on whether the claim had empirical support.