What Twin Studies Tell Us about Homosexuality: Nature vs. Nurture

What Twin Studies Tell Us about Homosexuality: Nature vs. Nurture

James R. Aist

(Note: Numbers in parentheses refer to specific, numbered references listed at the end of the article.)

Introduction

I presented a much broader treatment of possible causes of homosexuality elsewhere (click HERE). Most of the more recent research on possible biological origins of homosexuality has focused on the degree to which genes, along with other prenatal factors such as hormones and epi-genetics, may influence the development of homosexuality. In this regard, the most conclusive and telling results have come from studies of “identical” twins (who both have virtually the exact same complement of genes). Although it is commonly assumed that identical-twin studies reveal the influence of genes per se on a trait or behavior, the results of such studies have the unique advantage of reflecting, in fact, the combined influence of all possible, pre-natal, biological factors (e.g., genetics, epi-genetics, hormones, etc.) on the development of homosexuality in adults (1). This is because identical twins not only share the same complement of genes, but they also share the same pre-natal environment (their mother’s womb), where biological factors are postulated to operate.

Twin Studies: Overview

The design of research studies using identical twins has improved greatly since the mid-1990s, with the advent of large, twin registries which can afford much larger data bases and less biased sampling procedures. The former approach of recruiting identical twins via advertisements in gay and lesbian publications is now known to have a very strong “volunteer effect” that produced the appearance of relatively large genetic effects (1). Nevertheless, even with the use of large twin registries, the number of identical twin pairs found with homosexuality is often very small in individual studies, resulting in a standard deviation that is greater than the calculated genetic effect, meaning that the results are not statistically different from zero. In other words, the genetic influence or contribution in several of these studies may actually be zero, making definitive conclusions impossible. Whitehead and Whitehead (1) have presented and discussed, in some detail, these and other problems inherent in twin studies of homosexuality and have presented reasons to expect that the genetic influence on, or contribution to, homosexuality will eventually be agreed to be in the 10%-15% range (i.e., weak). One reason for this (anticipated) lower actual genetic influence is that epi-genetic effects operating through identical twins sharing one placenta probably represent about 15% of the total influence that has been attributed to genetics in published twin studies (2). Another reason has to do with the apparently predominant influence of post-natal environmental factors on the development of homosexuality (10). Schumm (9) found that children with homosexual parents are 12-15 times more likely than children of heterosexual parents to be homosexual as adults. This is the strongest environmental influence ever reported for the development of homosexuality, and it involves very close family members, the parent-child relationship. In a family environment, identical twins share a common bond and common experiences more so than do other siblings, including non-identical twins; siblings can’t be any “closer” than that. The results of a study by King and McDonald (8) illustrate how such a close, family relationship could inflate the calculated genetic influence on homosexuality in identical twin studies. They studied 46 twin pairs having homosexuality present in one or both of the twins in each pair and found that 54% of the twin pairs had discussed their sexual orientation with each other, 89% had “shared knowledge” of each other’s sexual orientation, and 30% of these twin pairs had actually had sex with each other. Because identical twins identify so closely with each other, and post-natal experiences  – especially close family relationships – strongly affect the development of homosexuality, it seems plausible, if not likely, that a homosexual member of a twin pair would influence the other member of that pair to embrace and explore homosexuality also, thus inflating the apparent genetic influence reported in identical twin studies. That is to say, a significant portion of what may appear, in identical twin studies, to be a genetic influence on the development of homosexuality may turn out to be, instead, a post-natal, environmental influence involving shared knowledge of sexual orientation and shared sexual experiences within identical twin pairs.

Twin Concordance Studies

The “pair-wise concordance” answers the simple question, “Where one twin of an identical pair is homosexual, what percentage of co-twins is also homosexual”. The mathematical formula for pair-wise concordance of identical twins is C/C+D, where C is the number of concordant (similar) twin pairs and D is the number of discordant (dissimilar) twin pairs found in the study. For example, if C=1 and D=9, then the pair-wise concordance would be 1/1+9=1/10 or 10%. This result would indicate that for every twin pair with both members being homosexual, there are 9 twin pairs with only one homosexual member.

Using data provided in several reports of large, twin registry studies in different countries, I performed a meta-analysis and calculated the range of pair-wise concordance to be 9.9% to 31.6%, with the average being 13.0% for males, 13.3% for females, and 13.2% when the raw data for males and females were combined. These pair-wise concordance values indicate that for every twin pair with both members being homosexual, there are 7 twin pairs with only one homosexual member. Now, compare this result to the range of theoretically possible outcomes where no twin pairs would both be homosexual (= 0%) and where all twin pairs would both be homosexual (= 100%) and you can see, intuitively, that a pair-wise concordance of only 13.2% would indicate a real, but relatively minor, contribution of genetics to homosexuality. This minor role is similar to the estimated level of genetic contribution to virtually any kind of human behavior (3) and is known to be non-determinative and, in many cases, treatable by therapy and/or counseling. For instance, the best example to date of a genetically related behavior (mono-amine oxidase deficiency leading to aggressive behavior) has shown itself remarkably responsive to counseling (3). Therefore, on the basis of pair-wise concordance in identical twins, it seems appropriate to conclude that there is, at the most, only a minor genetic contribution to the development of homosexuality, and that this relatively minor influence can be overcome (i.e., nullified) through behavioral therapy (1), which we know to be a fact (4, 5).

The other measure of concordance in twin studies is “proband-wise” concordance. This estimate of concordance is necessary in order to use both identical and fraternal twins in a study to disentangle the relative contributions to homosexuality of genetic and non-genetic (environmental) factors. The formula used is 2C/2C+D, which, compared to the formula for pair-wise concordance, gives much more weight to the individual twins (probands). The effect is to greatly increase, relative to pair-wise concordance values, the apparent genetic contribution to homosexuality in identical twin studies. To illustrate this point, if we use the example given above where the pair-wise concordance calculates to be 1/10 = 10.0%, the proband-wise concordance calculates to be 2/11 = 18.2%. Although it is less intuitive, proband-wise concordance is generally believed to give a better overall estimate of “genetic influence” than does pair-wise concordance.

Classical Twin Studies

While pair-wise concordance gives an intuitive indication of the genetic influence on homosexuality as expressed in identical twins, it does not provide information on what factors may provide the remaining, non-genetic influence. To answer this question, researchers are using other measures, broader-ranging questionnaires and more sophisticated statistical procedures to evaluate such things as heritability, additive genetic effects and postnatal environmental influences. In order to be able to put the results of classical twin studies into perspective, it is important to keep in mind that, by convention in the twin study literature in general, a genetic contribution of around 25% is considered weak, of around 50% is considered moderate and of 75% or more is considered strong (6).

In a meta-analysis, Whitehead (6), using the results from seven of the recent twin registry studies that were designed to reveal contributions of both genetic and non-genetic factors to homosexuality, found that the mean contribution of genetics to male homosexuality was around 22%, and to female homosexuality, around 33%. Because of the relatively large standard deviations in the data, these two values were not statistically different from each other.  Thus, the mean genetic contribution to male homosexuality in these studies is weak and to female homosexuality is weakly moderate. Such levels of genetic contribution indicate a real but weak-to-weakly moderate and indeterminate role of genetics in the development of homosexuality. For comparison, other traits that have around 50% (moderate and indeterminate) genetic contribution in twin studies include such things as divorce and alcoholism, while puberty has a 90% (strong and determinate) genetic contribution (1). Furthermore, the non-shared, post-natal environmental contribution to homosexuality is moderate to strong, around 64%-78%, has a relatively small standard deviation and is consistently around the same percentage (6), indicating that homosexuality is influenced primarily by post-natal environmental factors and experiences that are not directly related to prenatal, biological contributions of any kind or combination.

The recent study by Zietsch, et al. (7) can be used to illustrate representative research results obtained with large samples from twin registries. They used a very large sample (9,884) of twins from the Australian Twin Registry, one of the largest samples to date for twin studies of homosexuality. In this sample, there were 1,840 identical twin pairs (1,133 female and 707 male). Their calculated value of only 24% for the proband-wise concordance for homosexuality indicates a weak genetic influence. Moreover, their calculated figure of 31% for heritability of homosexuality also indicates a weak genetic component. This leaves around 68% of the variance represented by post-natal, “shared environment” and “residual” environmental influences combined.

Summary

In view of the fact that twin studies have shown that the combined influence of all possible, pre-natal, biological factors (e.g., genetics, epi-genetics, hormones, etc.) on the development of homosexuality in adults is only weak-to-moderate, it is important to understand that all of the biological theories combined can address only this weak-to- weakly moderate amount of influence, while ignoring the far more important post-natal influences (e.g., culture, parental divorce, and having a homosexual parent). Furthermore, twin studies clearly support the inference, based on results obtained through therapy and counseling (4, 5), that post-natal, environmental influences have a far greater role in the development of homosexuality than do pre-natal, biological influences. Thus, where the development of homosexuality is concerned, twin studies have demonstrated that nurture is far more important than nature.

(For more articles on HOMOSEXUALITY, click HERE)

References Cited:

1. Whitehead, N. and B. Whitehead. 2016. Chapter 10. Twin studies: The strongest evidence.(click HERE)

2. Whitehead, N. and B. Whitehead. 2016. Chapter 1. Can genes create sexual preferences? (click HERE)

3. Whitehead, N. and B. Whitehead. 2016. Summary. (click HERE)

4. Whitehead, N. and B. Whitehead. 2016. Chapter 12. Can sexual orientation change? (click HERE)

5. Aist, J. 2012. Homosexuality: Good News! (click HERE)

6. Whitehead, N.E. 2011. Neither Genes nor Choice: Same-sex Attraction is Mostly a Unique Reaction to Environmental Factors. Journal of Human Sexuality 3:81-114. (click HERE)

7. Zietsch, B., et al. 2012. Do Shared Etiological Factors Contribute to the Relationship between Sexual Orientation and Depression? Psychological Medicine 42:521-532.

8. King, M., and E. McDonald. 1992. Homosexuals who are Twins. British Journal of Psychiatry 160: 407-409.

9. Schumm, W. 2010. Children of Homosexuals More Apt to be homosexuals? A Reply to Morrison and to Cameron Based on an Examination of Multiple Sources of Data. Journal of Biosocial Science 42:721-742.

10. Aist, J. 2012. Are Homosexual People Really Born Gay? (click HERE)

Abortion: Biology, Bible and Forgiveness

A 3D ultrasound taken of a fetus at 17 weeks.Abortion: Biology, Bible and Forgiveness

James R. Aist

 “I was young and now I am old,
yet I have never seen the righteous forsaken
or their children begging bread.
They are always generous and lend freely;
their children will be a blessing.” (Psalm 37:25-27)

Introduction

There is no moral issue that grieves and saddens me more than abortion. I believe that abortion is, perhaps, the most egregious example of man’s inhumanity to man. But I am not writing this article to preach, judge or condemn. My purpose and motivation are to have a positive influence, to help save the life of at least one unborn child and to encourage at least one regretful mother to find forgiveness and peace.

Biology

When I took my first formal course in high school Biology in about 1960, everyone seemed to know that human life begins at conception, the union of a man’s sperm with a woman’s egg. Without uniting in the womb, the egg and sperm have no human life at all and do not develop into anything; they remain just an egg and a sperm and soon die. The part of the human “life cycle” that we universally agree includes all of the developmental stages of human life from babies to teens to adults actually begins with conception in the mother’s womb. Those are the basic, biological facts of human reproduction. So, from a biological perspective, human life begins at conception and proceeds on an uninterrupted continuum of developmental stages, first in the womb, and then outside of the womb. There is no identifiable point in this continuum at which an unborn child becomes suddenly human; the unborn child is a human being all along. The idea that there is a moment when a developing child changes suddenly from non-human to human is a fantasy that has no basis whatsoever in the science of biology. Rather, it is purely a figment of the human imagination, designed to rationalize the termination of innocent human life in the womb and, thus, avoid the enormous emotional toll that routinely follows in the aftermath of an abortion.

Ultrasound images or videos — especially the new 3-D versions, see above — of a developing child in his/her mother’s womb will provide conclusive and convincing evidence that the unborn child is, indeed, a human being in the early stages of development.

Bible

To receive the guidance of the God of the Bible – the Creator of human life — let’s have a look at some Scripture passages relating to abortion:

“Surely I was sinful at birth, sinful from the time my mother conceived me.” (Psalm 51:5)

How could David have been sinful before he was a person? If he was sinful from the moment of conception, then he must also have been a person from the moment of conception!

  • “Before I was born the Lord called me; from my mother’s womb he has spoken my name.” (Isaiah 49:1)
  • “I have been a Nazarite dedicated to God from my mother’s womb.” (Judges 16:17)
  • “… God… set me apart from my mother’s womb and called me by his grace…” (Galatians 1:15)

God has a plan for each of us, before we are even born! What is God’s plan for your unborn child?

  • “So God created mankind in his own image, in the image of God he created them….” (Genesis 1:27)
  • “Whoever sheds human blood, by humans shall their blood be shed; for in the image of God has God made mankind.” (Genesis 9:6)

Abortion is an attack on the very image of God himself.

I would guess that very few people are aware that the Bible has stern rebukes and dire warnings for those who approve of sin or encourage others to sin, but it does (Leviticus 19:1; Isaiah 5:20; Malachi 2:17; Matthew 5:19-20; Matthew 18:6; Romans 14:22). Thus, anyone, including born-again Christians, who even approves of or encourages the sin of abortion will, someday, have to answer to God for it. If that’s you, then now is the time to repent!

Forgiveness

Abortion is, first and foremost, a sin against God (see above). So, forgiveness and peace will come only through and from Him. Here’s how you can receive God’s forgiveness and peace:

  • Repent (turn away from your sins): “The Lord … is patient with you, not wanting anyone to perish, but everyone to come to repentance.” (2 Peter 3:9)
  • Believe and Trust in Jesus: “For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life.” (John 3:16)
  • Accept His Forgiveness: “Therefore, there is now no condemnation for those who are in Christ Jesus.” (Romans 8:1)
  • Receive the Peace of Christ: “Therefore, since we have been justified through faith, we have peace with God through our Lord Jesus Christ.” (Romans 5:1)

It’s important to keep in mind that there is no sin that is so big and so bad that God will not forgive it. In fact, God wants to forgive you and bring you peace. He has made this promise to you: “If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness.” (1 John 1:9). He is “faithful” to forgive you because He has promised to do so, and God does not lie or break His promises (Numbers 23:19); He is “just” to forgive you because Jesus paid the full price for your sins when He suffered and died for them, in your place, and, therefore, it would be unjust to not forgive you. And He doesn’t stop there. He will also cleanse you from the stain of your sin and of the guilt and shame associated with it; God will separate your sins from you “as far as the East is from the West” (Psalm 103:12), and he will not keep bringing it up and holding it against you (Isaiah 43:25). You have God’s word on it (Numbers 23:19), so take Him at His word, receive His forgiveness and move on with the peace of Christ; God has a wonderful plan for your life.

An Appeal for the Rights of the Unborn

If you want to make me cry, then remind me of the accidental traffic death of my daughter, Liesel, when she was only 20 years old. Or, remind me of the accidental drowning deaths of twin toddlers in Knoxville a few days ago. But, if you really want me to weep and mourn deeply, then remind me of the intentional deaths of tens of millions of innocent, helpless, unborn babies, murdered in their mother’s womb, in America since Roe v. Wade. The United States will have no claim whatsoever to any moral high ground concerning anything, unless and until we, as a nation, rise up against this, the most despicable of man’s inhumanity to man, and legally declare it to be what it is: murder of the worst kind. And that’s why we need more and more pro-life justices, judges, governors, legislators, presidents and God-fearing voters to lead the charge against abortion. America, bless God and stand against this national tragedy!

(For more articles on ABORTION, click HERE)