Genetic Research

What makes you the person who you are?  What caused you to develop your unique personality, a constellation of talents and interests, and a certain way seeing the world?  Broadly speaking, we can trace all of our influences to two sets of causes: genesand the environment.  We inherit genes from our parents, and this DNA serves as the building blocks of our brains and bodies.  On the other hand, the environment encompasses all of the experiences you've had in your lifetime - from the way your parents raised you to the education you receive at school.
Try it Out

Take a moment to reflect on your own personality, interests, and abilities.  Do you think you have been mainly shaped by genes, or by the environment?  For each of the characteristics below, indicate whether you believe that:

(a) genes are almost entirely responsible

(b) genes are mostly responsible

(c) genes and the environment are equal responsible

(d) the environment is mostly responsible

(e) the environment is almost entirely responsible

Ready?  Write the letter a-e for each of the characteristics below

1.  Your ability to sing ___

2.  Your aptitude for numbers ___

3.  How comfortable you are taking risks ___

4.  Whether or not you like spicy food ___

5.  Your weight ___

6.  Your favorite genre of music ___
Nature vs. Nurture Debate

The question of whether our personality is determined by genes or by the environment has ancient roots in philosophy.  In the 17th century, the French philosopher Descartes argued that certain ideas and character traits are inborn and already present at birth.  Differences between people can thus be explained by different genetic makeup.  On the other hand, British philosopher Locke criticised Descartes' ideas, arguing that we are born with a "blank slate" - a mind with no pre-existing ideas or personality traits.  Instead, it is our environment that determines what sort of person we become.  

The opposing views of Descartes and Locke gave birth to the "nature vs. nurture debate", one of the most enduring debates in the history of philosophy.  Many thinkers have offered reasons or arguments why either nature, or nurture, is more important.  As modern Psychology developed, many scientists began to explore how the scientific method could be applied to determine whether genes or the environment play a greater role.  Some of the most interesting research involved a special accident of nature - the unique case of identical twins.
Descartes - The mind contains inborn ideas at birth
Locke - At birth, the mind is a "blank slate" - everything comes from experience
Video Activity

Imagine that one day you come home from school and your parents confess a dark, family secret - you have a long-lost identical twin!  Your twin was somehow mixed up in the hospital at the time of birth, and was adopted by another family. 

Your identical twin comes from the same fertilized egg as you, and thus shares 100% of your DNA - in other words, your twin is your genetic clone.  But everything else about your life story is different.  Rather than being raised in a comfortable, middle class household, your twin was brought up by a family of travelling circus performers.  Instead of learning reading and writing, your twin learned fire-breathing and acrobatics.  Your twin grew up surrounded by knife throwers, magicians, and jugglers for companions.

Now imagine that you finally met your long-lost twin.  Would there be any similarities between the two of you?  In what ways would you expect to be similar, and in what ways would you expect to be different?

As far-fetched as this story may seem, there have been a number of documented cases not too different from this.  Cases where identical twins were separated as infants, only to be reunited years later.

Watch the video below, and make notes on the remarkable similarities that were observed between long-lost identical twins.  What does this suggest about the role of genetics in behavior?  How relevant are these case studies in resolving the nature vs. nurture debate?
Twin Studies

Twins provide a fascinating way to investigate the role of genes in behavior.  There are two types of twins, with different levels of genetic similarity.  Monozygotic twins (MZ twins) share 100% of their genes, as they come from the same egg.  Dizygotic twins (DZ twins), on the other hand, only share 50% of their genes, the same level of genetic similarity that exists between any non-twin siblings. 

Twin studies measure the concordance rates of a particular trait across the two different types of twins (and perhaps across other family relationships as well).  The concordance rate refers to the chance that one twin will have a particular trait, given that the other twin has it.  For instance, the concordance rate for schizophrenia is around 50% for MZ twins, but only around 17% for DZ twins.  That means that if you have an identical twin with schizophrenia, you have a 50% chance of also having the disorder, but that falls to 17% if it is your DZ twin that has the disorder.

If there is a significant difference between concordance rates for MZ and DZ twins, then it is thought that genes are responsible for the difference.  It is assumed that MZ and DZ twins are both brought up in an equally similar environment - for instance, raised by the same parents, grow up in the same house, go to a similar school, and so on.  Therefore, if MZ twins have a significantly higher concordance rate for a particular trait than DZ twins, it must be that extra 50% of genetic similarity that is responsible.

Although twin studies reveal the power of genes, in most cases they also hint at the role of the environment.  As mentioned, the concordance rate for schizophrenia is about 50% for MZ twins, far below 100%.  Even though identical twins share the exact same genes, and usually grow up in the same family, with the same parents, and in a similar social environment, there is still only a 50% chance that one twin will develop schizophrenia if the other twin has it.  So genetic factors alone can't predict who will develop a psychological disorder - we need to consider the environment as well.  That suggests the importance of life events and environmental circumstances in determining one's life outcome.

Limitations of twin studies

Although twin studies are very useful, they do suffer from a number of limitations.  These must be kept in mind when interpreting the results of twin studies.

  • MZ and DZ twins may not grow up in equally similar environments.  Twin studies are based on the "equal environments assumption" - that twins (both MZ and DZ) share roughly the same environment.  But that is probably not true.  Because MZ twins share 100% of their genes in common, they tend to look almost exactly the same.  DZ twins may look quite different, and in fact may even be of different genders.  Therefore, MZ twins may grow up being treated very similarity by parents, teachers and friends - perhaps being given the same toys to play with, joining the same after-school clubs, even sharing the same clothes.  DZ twins, especially if they are of different genders, will likely play with different toys, have different groups of friends, and participate in different social activities.  If the environment is more alike for MZ twins than DZ twins, then we can't be sure that genes are solely responsible for the higher MZ concordance rates.

  • Genes and the environment affect each other. Most twin studies operate on the assumption that genes and the environment are two separate sets of factors.  However, an increasing body of research suggests that genes and the environment influence each other.  For example, a gene might increase the risk of becoming violent, but only if you grow up in an abusive household.  Or a gene that makes you cry constantly as a baby may cause your parents to become irritable, treating you differently than they would a baby who sleeps quietly - and this difference in parental treatment could then influence your personality.  Twin studies are not designed to take this kind of complication into account.

Think Critically

A large number of twin studies have been carried out to investigate the role of genes in outcomes such as intelligence (IQ scores), aggression, and sexual orientation, amongst many others.

  • Using Google (or Google Scholar), find a twin study on one of the topics mentioned above.

  • For the study you have found, what is the concordance rate for MZ and DZ twins?

  • Does this study suggest that genes play a signficant role in the behavior? Explain why.

  • Does this study suggest that the environment plays a significant role in the behavior? Explain why
Video Activity

Modern research suggests that the entire nature vs. nurture debate may have been missing the point.  It isn't a question of whether it is nature or nurture that is responsible for behavior, but about understanding how nature and nurture interact.

Watch the video below on epigenetics, and answer the following questions:

1.  What is epigenetics?

2.  How do methyl groups influence gene expression?

3.  What determines whether methyl groups are removed in the brains of rats?

Epigenetics, put simply, is the study of how the environment affects gene expression.  Depending on your life experiences and environment, different genes can become turned "on" and "off", determining which genes take effect.  One way this happens is through methyl groups, which wrap around a gene and prevent it from being expressed.  Environmental factors can cause these methyl groups to become removed, leading to the gene being switched "on".  But without a particular set of environmental factors, the methyl groups may remain in place, and the gene remains switched "off".

An example might help to explain this.  We know that certain genes increase the risk of  developing depression.  But not everyone with risky genes ends up becoming depressed.  Much depends on the environment and life experiences - these determine whether the genes for depression become switched "on" or stay "off".  For example, if someone with risky depression-prone genes has a loving family and close friends who provide warmth, intimacy, and social support, that person may never become depressed.  On the other hand, a person with those same depression-prone genes who suffers from difficult life experiences - like neglectful parents, bullying classmates, or a painful divorce - may very well end up suffering a severe bout of depression.

Epigenetics is still a relatively new field, and biologists are still studying how different life experiences can turn particular genes "on" and "off".  What we do know for certain is that the field of epigenetics has turned the old "nature vs. nurture" debate on its head.  Genes and the environment are no longer considered two separate sets of factors that influence behavior.  Rather, genes and the environment continuously interact and influence each other, in turn shaping the unique person that you become.  Take a look at the video below, which gives some more examples of gene-environment interaction.


  • I can summarize the "nature vs. nurture" debate in philosophy

  • I can explain how twin studies are carried out to investigate genetic influences in behavior

  • I can discuss some of the limitations of twin studies

  • I can define epigenetics, and explain how methyl groups can influence which genes "turn on" or "turn off"

  • I can discuss the implications of epigenetics on the "nature vs. nurture" debate
Quiz Yourself!

1.  What sentence describes the implications of Locke's view that humans are born with a "blank slate"?

(a) A combination of genes and the environment determine personality

(b) Certain ideas - like reason, language and beauty - are biologically imprinted in the human mind 

(c) The environment is the sole determinant of one's outcome in life

(d) It is impossible to know what babies are thinking about

2.  A twin study is carried out on alcohol abuse, and finds that the concordance rate for MZ twins is 57%, while the concordance rate for DZ twins is 22%.  Which of the following conclusions are not valid?

(a) There appear to be genetic factors that contribute towards alcohol abuse

(b) The higher concordance rate for MZ twins can be explained by the fact the MZ twins have identical genes

(c) Both genes and the environment play a role in alcohol abuse

(d) A genetic test could be used to determine who will become an alcohol

3.  Twin studies are based on the "equal environments" assumption.  Why is this assumption likely false?

(a) Both MZ and DZ twins may not always go to the same school

(b) Parents and teachers often separate MZ twins so they have an opportunity to each develop their unique personality

(c) Because MZ twins look identical, they are more likely to be treated identically

(d) Because MZ twins share 100% of their genes, while DZ twin only share 50% 

4.  How do methyl groups influence gene expression?

(a) Removing methyl groups cause a gene to be switched off

(b) Removing methyl groups cause a genes to be switched on

(c) Methyl groups are only expressed in MZ twins

(d) Methyl groups are only expressed in DZ twins

5.  Epigenetics is the study of how

(a) Genes are transmitted from parent to child

(b) Genetic mutation contributes to psychological disorders

(c) Methyl groups play a significant role in depression

(d) Environment factors influence gene expression


1 - C, 2 - D, 3 - C, 4 - A, 5 - D