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:
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.)
- 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.