Difficulty finding credible statistics on incest percentages

Several times in online discussions, both on this forum and in other venues, I have been challenged by someone who believes that consanguineous relationships are very rare, very harmful, and very socially irresponsible. They usually refute whatever consanguineous-tolerant remark I post with something like, "Show me some believable statistics, then maybe you'll have some credibility."

So, I've spent some time trying to relocate articles that had statistics in them. One such article, which I have (unsuccessfully) spent quite a lot of time and energy trying to find, was the report of a survey poll of random university students, on a college campus, that had a staggeringly high percentage of college kids (especially young women) reporting sexual contact with a relative--as I recall it was 11% of total respondents. Such a study is not considered scientific, because it allows respondents to self-select. Some people would refuse to participate in the survey who actually were involved in incest, and others might falsely answer "yes," as a prank.

Another poll I read reported percentages of babies born and anonymously tested for DNA birth defects, which supposedly revealed that 2.3% of the babies tested had homozygotic DNA (DNA where both parents shared DNA themselves, like father-daughter or brother-sister.)

Accurate information on this topic is very hard to come by. You'd think that you would be able to type "What percentage of Americans are incestuous?" into your browser and bing-bang-boom, out comes the answer. It's not that easy.

In a survey of women obtaining an abortion, one-tenth of one percent reported they believed the baby was a product of incest and were worried it would have birth defects.

So if we applied these widely varying results to the overall population:

315 million x 0.11= 34,650,000. This seems way, way too high to me.
315 million x 0.023= 7,245,000 This seems much more likely.
315 million x 0.001= 3,150,000 This seems too low, especially since the pregnant women were all seeking to abort the fetus, indicating an unhappy, negative outcome of the incest.

This article, below, while very condemning of incest in general does seem to have some believable statistics. Here's a pertinent paragraph.

Incidence and Prevalence of Incest

Until the late 1970s and early 1980s, incest was believed to be exceedingly rare. It has proven difficult to accurately estimate the incidence (the number of incestuous cases that have occurred over a period of time) and prevalence (the number of people who have been victims during their lives) of incestuous abuse. There are many reasons why it is difficult to obtain accurate estimates. These include difficulties in defining incest (some definitions list only sexual intercourse as incestuous abuse, while others include other types of sexual contact). In addition, many victims of incest may fail to report the act, a tendency that incest shares with rape. However, various studies place the percentage of incest victims in the general population of America at about10 to 20 million. Among the reported victims of incest, girls outnumber boys by approximately ten to one; however the number of boys abused may be underreported.

Read more: http://www.faqs.org/health/topics/68/In ... z3OpwRFTUk
http://www.faqs.org/health/topics/68/Incest.html

Keep in mind, the above figure ("10 to 20 million") is an extrapolation that only includes incest where one of the people involved is considered a victim. Situations where nobody reports it and does not consider that anybody has been victimized (e.g. where both parties are not distressed or at least, not sufficiently distressed to report the incest) are not included in the extrapolation.

I'm not sure that replying to one's own post is considered acceptable here, but after reviewing the information in the post above, I realized that the figure that I thought to be "way, way too high" (34 million) is pretty close to the figure the article clip cites ("10 to 20 million" incest victims) if you double the number of "victims." Obviously, those 10 to 20 million "victims" must have a "perpetrator." Therefore the number of people who participated in an incestuous contact just doubled.
I believe these figures ("10 to 20 million" incest victims) are extrapolated from criminal cases and social services cases. It is impossible for the authorities to count each and every "incest victim," and it is clear they are using the number of "victims" who do show up in courtrooms and hospitals figured into the size of the population (315 million +/-) to arrive at a derivative likely number of victims. However SOME of that number are not showing up in the courts or hospitals because they are not "victims" at all, but consensual participants. The authorities insinuate that these people are victims, suffering in silence. I say that they are probably hiding from the District Attorney's office because they do not think they are doing anything that should be considered a crime.
But, if we use the lowest figure ("1/10 of 1%" of women seeking an abortion--a pretty SOLID number) that still indicates 3,150,000 X 2 = 6,300,000 Americans involved in incest. Mind you, this 1/10 of 1% were not necessarily seeking an end to the relationship that resulted in pregnancy, but only to terminate the pregnancy itself, and, presumably, to obtain an effective method of birth control.
I concede that my figures might be inaccurate. Anybody care to help make them more accurate?

I've only scanned through the above cursorily, but it looks like you're struggling with the stats. For example, you're taking 'diachronic' data and superimposing it on a 'synchronic' population figure. But if those incest 'victim' figures are over a period of time, then you have to take into account the fact that the population will change, new generations will emerge, old generations will die out. Furthermore, far from all incestuous experiences will result in a baby. Some will be fleeting (a drunken tryst, a short-lived affair), others will involve sufficient contraception, others will be homosexual. Etc. Statistics is a highly technical field. Of course, the difficulties you're faced with could be resolved if you did manage to find some decent studies authored by people familiar with the ins and outs of statistics.

Close incest is illegal, so no duh nearly everyone engaging in it voluntarily keeps quiet.

Personally, I don't think voluntary sexual acts between consenting adults is anyone's damn business.
As to the slightly increased risk of birth defects, that's a non-issue as no one opposes unrelated people with expressed handicaps boinking.