Twins on the average seem to have lower IQs than singletons. The best estimate for this group difference is 4.2 IQ points (less than one-third of a standard deviation), with a great divide between study outcomes of less vs more recent birth cohorts (5.1 vs 0.5 IQ points, respectively).
It was concluded, among many other things, that identical twins are about 85 percent similar for IQ, whereas fraternal twins are about 60 percent similar. This would seem to indicate that half of the variation in intelligence is due to genes.
Conclusions: Twins and triplets fall slightly behind singletons in cognitive and physical development, even in early adulthood, but twins and triplets show no major differences.
A new study shows that first-born children tend to be smarter than their siblings and second-born children are more likely to cause trouble. The University of Edinburgh study reported that the oldest child tends to have a higher IQ and thinking skills than their younger siblings.
Researchers have previously shown that a person's IQ is highly influenced by genetic factors, and have even identified certain genes that play a role. They've also shown that performance in school has genetic factors. But it's been unclear whether the same genes that influence IQ also influence grades and test scores.
Most twins, triplets and more grow and develop along roughly the same lines as their singleton peers – even those who start out much smaller will catch up in time. But there is nothing to say twins, triplets or more have to reach milestones at the same time as each other.
However the researchers looked at it, first-borns had, on average, an IQ of 1.5 points higher than second-born siblings, who in turn had a 1.5 higher IQ than third-borns and so on.
Unfortunately, they also share the risk of having ADHD. According to an Australian study , twins are more likely to have ADHD than singletons. Additionally, a child who has an identical twin with ADHD has a high chance of also developing the disorder.
Thayer's study shows that the ADHD group of children had larger and more frequent variations. Fathers with ADHD will pass this code discrepancy to offspring. Barkley explains that the heritability of ADHD runs around 80 percent. Genetics account for 80 percent of the components that define ADHD.
Twins and other multiple-birth individuals can suffer from much deeper and troubling loneliness than single-born individuals. Separation anxiety, which often begins at birth, is the underlying cause of loneliness for twins.
Statistics show twins or higher order multiples can take longer to reach milestones, especially those born prematurely. With the average pregnancy usually lasting 40 weeks, if twins were born at 35 weeks, their progress and milestones will be monitored as though they were five weeks younger than they really are.
At three months:
The three-month stage marks the end of the “fourth trimester” and your twins can better adapt to life outside the womb. For one thing, they'll sleep in longer stretches of about three- to four-hour chunks.
As expected, first-born twins had greater birth weight than second-born twins. With respect to height, first-born twins were slightly taller than second-born twins in childhood.
Using this more accurate method, researchers have found that birth order does not affect intelligence, and that differences in intelligence observed in previous trials are most likely due to external factors such as parents' intelligence or economic disadvantages more often faced by larger families.
Numerous studies have established that twins, triplets and other sets of multiples have a higher likelihood of speech delays. (It's also more common in identical twins and multiples than in fraternal.)
Twin Language Development
There is tons of variation in language scores among twins, but overall, research shows that twins do tend to fall behind in language a bit more than single-born children do. Male twins in particular are vulnerable, often falling about 6 months behind even female twins (Lewis & Thompson, 1992).
Thus identical twins, though they start with the same genes, likely develop different personalities in the same environment partially based on how they interact with their environment.
One placenta and one amniotic sac.
This is the riskiest and rarest type of twin pregnancy. Fetal complications can arise due to tangling of the umbilical cords or an imbalance in nutrients, blood or other vital life supporting systems.
Twins are always the same age, and they usually share a similar environment, both in the womb and while growing up. Identical twins share 100% of their genes, while fraternal twins share only about half their genes (just like regular siblings). The concept of concordance is important in twin studies.
Twins are more or less equally likely to be female or male.
People over 30, and particularly those over age 35, are more likely to conceive twins. 4 This is because the level of follicle-stimulating hormone (FSH) rises as birthing parents get older. FSH is responsible for the development of eggs in the ovaries before they are released.
The reality is that raising multiples is hard. You have double or triple the feeding, diapering, and laundry and, as a result, less time to spend cuddling and getting to know each baby. To be sure, there will be days when you feel as if you're walking up a down escalator.
Twins will have better communication skills and less sibling rivalry than other children, a study shows. 'Multiple' siblings will have closer bonds than siblings of different ages and are likely to be best friends until adulthood, experts say.
That's because twins are born prematurely 60 percent of the time and land in the costly neonatal intensive care unit more often than singletons. The article also mentions that these babies are more susceptible to mental retardation and learning disabilities.
It is likely that twins' awareness of one another starts sooner than seven or eight months of age. An article by the late doctor, T. Berry Brazelton, observed that at age three to four months, an infant identical female twin seemed disoriented when her sister was removed from the room.