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).
The mean absolute difference between twins is 6.60 (SD = 5.20), the largest difference being 24 IQ points. The frequency of large twin differences is no more than would be expected from the normal probability curve. The overall intra-class correlation between twins is.
For IQ scores, twins scored 0.09 points lower than non-twins at age 8 and 0.83 points lower at 10. However, twins scored higher at age 12 by 0.14 points. The only significant difference found was at age 10.
Because identical twins share all of their genes, their measurements of IQ and sleep time will be more similar the bigger role genes play in it (i.e., differences between two identical twins must be caused by the environment because their genes are the same).
If two twins in an identical pair tend to have closer IQ's than two twins in a fraternal pair, then genes probably play a role in IQ. And that is just what we see. Identical twins are definitely more likely to share the same IQ than are fraternal twins.
Identical (i.e., monozygotic, or MZ) twins share 100 percent of their genes, whereas fraternal (i.e., dizygotic, or DZ) twins generally share only 50 percent of their genes.
The DNA of monozygotic twins tends not to be 100% identical, and epigenetic and environmental differences further widen the gap between twin pairs. It's not nature or nurture; it's a complex interaction between our genes, our environment, and our epigenetic markers that shape who we are and what illnesses befall us.
But genetics can explain the wide range of possible IQs too because so many different genes are involved in developing and running a brain. It is possible, for example, to inherit all the higher IQ genes from each parent and leave the lower IQ ones behind. Now the child will be brighter than the parent.
The study appears in Behavior Genetics. ASD is a developmental disorder that affects how a person behaves, interacts with others and learns. Previous studies have found that when one identical twin has ASD, chances are extremely likely that the other twin has it, too.
Like most aspects of human behavior and cognition, intelligence is a complex trait that is influenced by both genetic and environmental factors.
Now a new study, published in the Proceedings of the National Academy of Sciences and based on data from 20,000 individuals, concludes that birth order does matter when it comes to IQ – with the oldest having slightly higher IQ than their younger siblings.
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).
The University of Edinburgh study reported that the oldest child tends to have a higher IQ and thinking skills than their younger siblings. This is due to higher mental stimulation the first-born receives, CBS affiliate KUTV reports.
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.
Because identical twins share all of their genes, their measurements of IQ and sleep time will be more similar the bigger role genes play in it (i.e., differences between two identical twins must be caused by the environment because their genes are the same).
“Twin studies of major depressive disorder have showed that if one twin has the illness, a fraternal twin has an increased likelihood of developing the disorder and an identical twin's likelihood is even higher,” says Dr. Avari.
Why? Because identical twins share the exact same DNA. In twin studies looking at schizophrenia, scientists have shown that if one identical twin has schizophrenia, then there is a 30-50% chance that the other twin will have it as well1.
The first genius IQ score was around 140. That's about one in every 250 people. But one leading researcher in the 1940s suggested that a genius should have an IQ over 180.
Empirical evidence suggests that especially parental education, parental income, and maternal IQ are important predictors of intelligence. Parental education together with maternal IQ and the child's sex were found to account for 24% of the variance in IQ at age 5 [6].
130 to 144: Moderately gifted. 145 to 159: Highly gifted. 160 to 179: Exceptionally gifted. 180 and up: Profoundly gifted.
Monoamniotic-monochorionic Twins
This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords. If you have monoamniotic-monochorionic twins, your healthcare provider will monitor your pregnancy closely.
They come from the same fertilized egg and share the same genetic blueprint. To a standard DNA test, they are indistinguishable. But any forensics expert will tell you that there is at least one surefire way to tell them apart: identical twins do not have matching fingerprints.
Since identical twins share the same DNA, the children of two pairs of identical twins are legally cousins, but genetically more similar to siblings.