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  • the likelihood of having identical twins (which happens when one fertilized egg divides in half) is holding steady at about 1 in 285.
  • The 2011 twin birth rate was 33.2 per 1000 total births, essentially unchanged from 2009 and 2010. The rate of twin births rose 76 percent from 1980 to 2009-2011. From 1980 to 2004, increases averaged nearly 3 percent a year (peaking at more than 4 percent from 1995 to 1998).
  • Women tend to wait until they get older to have children and at that point the hormonal changes make it more likely that your body will release more than one egg at a time.

Identical Twins

  • most of the rise in multiple births was due to the use of fertility drugs and assisted reproductive technology (ART) to help women conceive. These treatments greatly increase a woman's chance of having twins or higher-order multiples
  • one of a pair of twins who develop from a single fertilized ovum and therefore have the same genotype, are of the same sex, and usually resemble each other closely

Video

(1:33-4:40)

Nature vs. Nurture

  • In addition to a unique biological foundation (genetics), every person is also influenced by a multitude of environments: family, school, friends, neighborhoods, religion, and culture.

Nature vs. Nurture in Identical Twins

  • Genetics or nature influences almost every aspect of development, from personality and physical development to cognitive processes such as language development and intelligence

Cases in Which Identical Twins were Separated at Birth

  • Nurture is the total effect of all the external environmental events and circumstances that influence your development
  • statistics have shown that on average, identical twins tend to be around 80 percent the same in everything from stature to health to IQ to political views.

How Nature and Nurture Relates to Identical Twins

  • It is not really a case of nature or nurture. Rather, it is the interaction of these two forces that influences behavior.
  • Twins are useful in such studies because almost all twins share the same home environment as each other, but only identical twins share exactly the same genetics.
  • evidence from the comparison of twins raised apart points rather convincingly to genes as the source of a lot of that likeness

Jim Lewis &

Jim Springer

  • Recent advances in genetics, however, suggest that opposing “nature” to “nurture” is misleading. Genes combine with the environment to produce complex human traits.

Daphne Goodship & Barbara Herbert

In this study, researchers focused on the tale of identical twins who were separated at birth and adopted by different families. It wasn’t until they were 39 that they met, and they were surprised to find they both suffered headaches, bit their nails, smoked Salems, took up woodworking and even vacationed on the same beach in Florida. In fact, their adoptive families also coincidentally named each twin Jim.

  • evidence suggests that genes influence such traits and behaviors as height, weight, manic-depressive psychosis, schizophrenia, alcoholism, cognitive development, rate of accident occurrence in childhood, television-viewing habits, peer-group selection, timing of first sexual intercourse, marital disruption, and educational and economic attainment
  • Daphne Goodship and Barbara Herbert first met when they were 40. Debbie was raised Jewish and Sharon was raised Catholic.They admit that they’ve also cooked the same meal from the same recipe book on the same day, without knowing it. They also discovered that they both had a miscarriage in the same year.
  • Contrary to traditional views, environmental factors that twins share tend to play a much smaller role.
  • Environmental factors unique to each individual also seem important
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