Minnesota Twin Family Study
What's Special About Twins to Science?
By studying identical and fraternal twins and their families, we can estimate how genes and environment
interact to influence character, strengths, vulnerabilities, and values.
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Identical (MZ) twins are genetic duplicates.
Fraternal (DZ) twins share 50% of their genes.
Both types of twins share common life experiences.
These facts plus information we collect from you about how alike or different members of a twin
pair are allow us to determine how genes and environment influence...
o falling in love
o divorce
o aging
o personality development
o vocational choices
o talents and abilities
o attitudes
o body characteristics
o health
o coping with stress
o brain waves and other physiological responses
How Are Identical (MZ) and Fraternal (DZ) Twins Made?
Identical twins differ from fraternal twins in the number of genes they have in common.
Genes are the chemical units in the cells of the body.
Identical, or monozygotic (MZ), twins have the same genetic makeup.
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Shortly after a single egg cell has been fertilized by a single sperm cell, the fertilized egg cell (or
zygote) splits in half.
Rather than developing into one individual, the fertilized egg cell develops into two individuals
with the same genetic makeup.
Fraternal, or dizygotic (DZ), twins share 50% of their genes.
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Two different individuals, no more genetically alike than brothers and sisters develop from
separate fertilizations.
Can doctors tell if twins are identical or fraternal?
Most think that fraternal twins have two placentas and identical twins share one placenta, but this is not
the case.
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One-third of identical twins separate within a few days of conception, before the placental tissue
has begun to form. Each embryo then grow its own placenta.
So, if your doctor said that you and your twin were fraternal because you had two placentas, the
doctor may be incorrect.
This is a fairly common mistake with as many as twenty percent of all twin births resulting in
identical twins who think they are fraternal.
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This is one reason why we take special steps to determine zygosity, including sometimes taking
blood.
What Accounts for the Similarity among Twins, Genes, or Environment?
One way to answer this intriguing question is to compare MZ twins reared together with those separated
at birth and raised in different families.
Similarity in twins reared together is due to genes and environment.
Similarity in twins reared apart is due only to genes because they don't share experiences together.
Most people would guess that twins reared together would be more alike than twins reared apart. But
are they?
What's the answer?:
An MZ twin reared away from his or her co-twin seems to have about as good a chance of being similar
to the co-twin in terms of personality, interests, and attitudes as one who has been reared with his or her
co-twin.
This leads us to believe that the similarities between twins are due to genes, not the environment.
Because the differences between twins reared apart must be due totally to the environment, and
because these twins are just as similar as twins reared together, we can conclude that the environment,
rather than making twins reared together alike, makes them different.
What is this thing called LOVE?
How can Minnesota Twin Studies help us to determine what's behind "that thing called LOVE"? Why do
we feel that we have found our "One and Only"? Cupid's bow and arrow image may hold more truth than
you think. Look below to see what we have found with the help of Minnesota twins and their families:
Step #1:
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Some think that based on genes and unique experiences, we develop a set of criteria for the
mate that is right for us.
How could we test this theory?
We'd use twins, naturally. Identical (MZ) twins have the same genes and share much of the same
experiences. If this theory is correct, a pair of MZ twins should have very similar "mating criteria".
So, we would expect:
1. MZ twins to choose spouses that are very similar.
2. Spouses of twins should feel some attraction to the co-twin--above average for DZ twins
and a very strong attraction for MZ twins.
Step #2:
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What happens when we test the theory? The theory bombs!
1. The spouses of MZ twins are no more alike than the spouses of DZ twins and hardly more
alike than random pairs of people!
2. When we asked male twins how they felt about their co-twin's spouse, two-thirds of the
twins said they were indifferent to their twin's mate or actively disliked her!
Step #3:
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What does this mean?
Our choice of mate is one of the most important choices we ever make and yet it seems to be a
random affair, not strongly influenced by genes or upbringing. The person that we end up with is
determined by a kind of lottery, by whom we happen to be near when Cupid's arrow strikes!
Could Divorce be Inherited?
Can you figure out the connection between genes and divorce?
We have suggested that there is not a "divorce gene" but rather, the risk for divorce depends on many
psychological characteristics, all of which are genetically influenced.
Here's what we found:
By asking twins about divorces in their families, a divorce risk was calculated according to which family
member was divorced.
Minnesota Population Rate: About 19% of Minnesota couples have been divorced.
Twin Family Rate: About 20% of the twins had divorce in their families.
If Parent Divorced: The risk of divorce climbs to about 29% if your parents are divorced.
If Spouse's Parent Divorced: The risk of divorce is about 31% if your spouse's parent is divorced.
If Fraternal (DZ) Twin Divorced: If you are a DZ twin and your co-twin is divorced, your risk of divorce is
30%.
If Identical (MZ) Twin Divorced: ***If you are an MZ twin and your co-twin is divorced, your risk of
divorce to 45%, which is 25% above the rates for the Minnesota population. Since this was not true for
DZ twins, we can conclude that genes do influence the likelihood of divorce.
How do we explain these results?
We know from other studies that genes strongly influence personality characteristics.
Identical (MZ) twins thus have very similar personalities.
We believe their similar divorce rates are due to their having genetically influenced personality
characteristics that contribute to marital adjustment.
Growing Up and Growing Apart?
As twins grow older, do they become less similar to one another?
MZ (identical) twins become less similar in terms of physical traits like looks and weight over time.
However, we recently found that MZ twins become more similar with respect to abilities such as
vocabularies and arithmetic scores. Ultimately, twins like you will provide us with the answer to
questions about whether personalities, interests, and opinions of MZ and DZ twins become more or less
similar with age.
As DZ (fraternal) twins get older, as they leave their rearing home, marry, start careers and families, and
develop their own circle of friends, they become less similar with respect to vocabularies and arithmetic
scores.
Twins and Age of Death
The bar graph shown above demonstrates that MZ twins tend to have more similar ages at the time of
death than DZ twins do. That is, MZ twins are more likely to die at about the same age, and DZ twins
are more likely to die at different ages.
Resting EEG in Twins
What is EEG and why is it important to measure in twins?
An EEG is a measure of brain activity or brain waves. By measuring brain waves, we can monitor a
person's state of arousal, that is, whether he or she is asleep, awake and relaxed, or awake and excited.
This figure shows the different types of waves seen during different states of arousal.
Alpha waves are seen when one is awake and relaxed.
Beta waves are seen when one is awake and alert.
Delta waves are seen during sleep.
Theta waves are seen when one is drowsy.
Since identical (MZ) twins share all of their genes, and fraternal (DZ) twins share about 50% of their
genes, we can estimate how much genes influence states of arousal by determining how similar EEG is
for members of the two types of twin pairs.
It is difficult to determine twin similarity using squiggly EEG waves like those shown above. To simplify
our task, we can use a computer to draw a picture, called a spectrum, that tells us how much of each
type of EEG wave is present. Refer to the figures below to compare the EEG spectra of identical (MZ)
twins and fraternal (DZ) twins.
These spectra were obtained from twins who were relaxing with their eyes closed. As you can see, what
really makes the twins' spectra look the same or different is the amount of EEG alpha that is present.
Some twins produce a great deal of alpha, causing their spectra to have a peak. Others generate little
alpha and therefore have spectra that look relatively flat.
In this relaxed state, MZ twins tend to generate the same amount of alpha, producing strikingly similar
EEG spectra. DZ twins show far less similarity. If EEG is determined mostly by genes, this is just what
we would expect to see.
Brain Responses (P300) in Twins
What is a P300 and why is it important to study in twins?
A P300 is a tiny electrical response (a few millionths of a volt) that occurs in the brain when a person
detects something that is unusual or interesting. For example, if a person were shown nine circles and
one square, a P300 brain response would appear after seeing the square because it's different.
Like EEG, P300's are important to twin studies because we can assess what role genes or
neurophysiological factors (which twins share) play in determining how the brain processes information.
These results indicate that the way the brain processes information may be greatly influenced by genes.