As a psychologist with a special interest in twins and twin studies I receive a great deal of interesting information from people around the world. Here is a selective sampling of some of the recent twin-related items I have had the pleasure to read. An expanded version of this material will appear in a forthcoming issue of the journal Twin Research and Human Genetics.
World-Wide Twinning Trends. A paper reporting twinning rates across developing countries was recently published by J. Smits from the Netherlands, and C. Monden, 2011) from the United Kingdon. These investigators gained access to a most unusual data set. They obtained information on twinning among women (15-49 years of age) who were interviewed in 150 Demographic and Health Surveys across 75 low- and middle- income nations. Data for China were also included in the study, based upon published figures from the 1990 census. The data, gathered between 1987 and 2010, included only births occurring during the ten years prior to the interview to limit the time frame of the study and to avoid selection difficulties. That decision yielded 2,473,209 births involving 1,379,694 mothers. There were 30,895 multiple births.
Alow twinning rate of 6-9/1,000 was observed in East Asian countries and also characterizes the entire south and southeast Asian region. Nigeria turns out not to hold the highest record for DZ twinning as has been true in the past--that distinction now belongs to Benin whose twinning rate is 27.9/1,000. Twinning rates in Latin America are at the same low level as those in Asia. These regions may have unusually high identical twinning rates, up to 8/1,000, in which two-thirds of the twins born are identical.
The high twinning rate in Central Africa continues to pose serious health challenges, given the greater physical risks to which mothers and twins are subject. Identifying areas of increased twinning is vital for introducing health care programs and support services.
"Mixed Race" Twins. Eighteen-year-old James and Daniel Kelly, are part of the unusual subset of fraternal twins who appear to belong to different racial groups (Moorhead, 2011). James resembles their black Jamaican-born father and Daniel resembles their Caucasian British-born mother. Interestingly, each of their parents had had twin children in previous marriages-fraternal twin boys from their father, and identical twin boys from their mother.
University of Edinburgh population geneticist Jim Wilson suggested reasons for the twins' marked physical differences. He explained that European DNA was introduced into the Caribbean gene pool by slave owners who raped female slaves. As such, James would have inherited his father's dark skin variants, while Daniel would have inherited variants for lighter skin. Dr. Wilson believes that because the twins' father most likely has more African than European DNA, the chance of having a light-skinned child like Daniel was slim-only 1/500 sets of twins with such interracial parents should expect such different looking children. How prof. Wilson arrived at this figure was not provided.
The twins, who were raised in southeast London, were ostracized by classmates when they were young, but Daniel faced greater difficulty than his twin brother. In particular, he had been accused of inappropriately calling himself white despite his black heritage. Daniel also experienced insensitivity from his nursery school teacher who required that he draw a picture of himself with dark skin. Daniel's mother withdrew her sons from the school, saying that her son had a right to identify as white. She also felt that this situation made her feel irrelevant "as though my colour didn't matter."
Mixed-race twins are a fascinating, but understudied group of DZ twins. They are thought to be rare, but I suspect that a modest sized, but informative sample could be assembled, especially if individual cases were pooled. Such twins would highlight factors associated with identity and self-esteem, given that family background and other environmental factors are controlled.
Mysterious Languages. Ten additional "mysterious ciphers and languages" were recently recognized (Weidinger, 2011). Among them were the Ptolemy Map Code, the Feynman Ciphers, and Cryptophasia. Cryptophasia is the well-known private speech that may evolve between twins. Weidinger distinguished it from idioglossia because he asserts that the former also includes mirrored actions, such as gestures and mannerisms.
Weidinger correctly writes that twins or other children who display cryptophasia do not truly invent a language, but use each other as linguistic models more often than they do adults, creating a unique vocabulary between them. Such speech is often unintelligible to others. He also indicates that cryptophasia occurs among 40% of twins, but does not cite a reference. I believe the original source of this much-quoted statistic is Bakker (1987), a figure that requires current confirmation.
Piano Duo. Unusual musical talent is rare and greatly admired, but even more so when that talent is repeated by two individuals. Identical twins Christina and Michelle Naughton, from Madison, Wisconsin, are famous for piano duets, which they have performed all over the world (The Schubert Club, 2011). They began their musical studies at age four.
In October 2011 the Naughton twins returned to the Midwest to appear at the Ordway Center for the Performing Arts, in St. Paul, Minnesota. They close their pieces wisely, among them Andante and Allegro brilliant, Opus 92 (for Four Hands; Felix Mendelssohn); Variations on a Theme of Joseph Haydn, Opus 56b (for Two Pianos; Johannes Brahms) and Variations on a Theme by Pagonini (for Two Pianos; Witold Lutoslawski).
Bakker, P. (1987). Autonomous languages in twins. Acta Geneticae Medicae et Gemellologiae, 36, 233-238.
Moorhead, J. (2011). "Black and White Twins," The Guardian, http://www. guardian.co.uk/lifeandstyle/2011/seo/24/twins-black-white/print.
Smits, J., and Monden, C., (2011). Twinning across the developing world. http://www.plosoneorg/article/info%3Ado%2F10.1371%2Fjournal.pone.0025239.
The Schubert Club (September 13-October 31, 2011). "Christina and Michelle Naughton, piano duo," 16-17.
Weidinger, P. (2011). "Crime & Mystery: 10 More Mysterious Ciphers and Languages," http://listserve.com/page/4/.