Week 3- Twin studies Flashcards
What are twin studies?
- Identical (Monozygotic) twins provide a unique opportunity to investigate the role of genetic factors on differet traits.
- 100% of their DNA is identical
- Non-Identical (dizygotic) twins share approx. 50% of their DNA, the same as any other sibling.
- So, if personality/intelligence/physical or mental health disorders are solely determined by genetics, then identical twin studies can provide evidence for this.
What did FRANCIS GALTON (1874) state?
- There are twins of the same sex so alike in the body and mind that not even their own mothers can distinguish them
- Their features, voice, and expressions are similar
- They see things in the same light, and their ideas follow the same lows of association
- This close resemblance necessarily gives way under the gradually accumulated influences of differences of nature, but often lasts till manhood
Thorndike (1905) carried out the first psychological tests utilising twins
- used 50 pairs of twins to explore why twins resemble eachother in regards to mental traits
What was Thorndikes argument/theory?
If resemblance is due to environment (treated alike home, have the same parental models, attend the same school)
1. Resemblance should be no greater among twins that among siblings who are reared in the same environment
2. Twins should up to the age of leaving home, grow more and more alike
3. Resemblance should be greated on tasks that are subject to training (i.e. addition/multiplication) compared to more novel tasks
- Resemblance no greater among twins
Twins and siblings were given a number of mental reasoning tests:
- A Test: Find and identify the number of “As” distributed amongst other capital letters.
- Word Test: Find and identify words that contain two specified letters (e.g. e & r).
- Opposite Test: Writing a word that is the opposite in meaning of a stated word
- Twins should grow more and more alike
Twins aged 9-11 were compared to twins aged 12-14 on a number of tests
- A Test: Find and identify the number of “As” distributed amongst other capital letters.
- Word Test: Find and identify words that contain two specified letters (e.g. e & r).
- Opposite Test:Writing a word that is the opposite in meaning of a stated word.
- Misspelled Word Test: Finding and marking misspelled words on a page of words.
- Addition:Efficiency in adding numbers.
- Multiplication: Efficiency in multiplying numbers.
- Resemblance greater on trained task
Twins aged 9-11 were compared to twins aged 12-14 on a number of tests;
Tests subject to most training:
- Addition: Efficiency in adding numbers
- Multiplication: Efficiency in multiplying numbers
Tests subjects to least training:
- A Test: Find and identify the number of “As” distributed amongst other capital letters.
- Word Test: Find and identify words that contain two specified letters (e.g. e & r).
Tests moderately subject to training::
- Opposite Test: Writing a word that is the opposite in meaning of a stated word.
- Misspelled Word Test: Finding and marking misspelled words on a page of words.
What are the CONCLUSIONS and LIMITATIONS?
CONCLUSION:
- None of the 3 claims examined provide strong evidence that environment is responsible for resemblance in mental traits among twins
- The trained vs novel tasks provides some evidence that trained tasks have greater resemblance among twins
- BUT… it is the middle group of tasks - moderately trained 0 that have the greatest resemblance
- Genes cause similarity and difference; environmental impact is slight
LIMITATION:
- in 1905 it was not understood that there were identical and non-identical twins.
- So it is unclear which type of twins made up Thorndikes sample
CURTIS MERRIMAN (1924)
There are two distinct type of twins, FRATENRAL and DUPLICATE
Fraternal:
- Being of two egg origin
- Should show no greater resemblance than ordinary siblings, since each individual of the pair develops from a wholly independent arrangment of the factors for heredity in the germ cells
Duplicate:
- Being of one-egg origin
- Should show a very much higher degree of resemblance than the fraternal because each member of the pair develops from substaintially the same arrangement of the factors for heredity in the germ cells
HOWEVE…. Merriman did not highlight how twin studies could be used effectively examine the influences of gene/environment on traits…
- Instead, he took the existence of 2 types of twins as further evidence that genes are the powerful deciding factor in human abilities and the impact of the environment is minimal.
- He did not identify a group of identical twins vs non-identical twins to
compare. - Tallman (1928) and Wingfield (1928) were the first to compare Identical vs non-identical twins in relation to IQ
Identical twins and intelligence
If there are twins with the exact same genes, this allows for a different type of study to look at the impact of genes vs the environment
- Studies which compared identical twins reared in the same environment vs identical twins reared apart
- Cyril Burt (1966) carried out such research lookin at intelligence
What did Burt’s research conclude
The correlation between intelligence of MZ twins is significantly higher than the correlation of siblings, and the environment has minimal impact
CURRENT VIEW: Sternberg and Grizgorenko (1997) - virtually all researchers accept that:
- Both heredity and environment contribute to intelligence
- Heredity and environment interact in various ways
- Extremely poor/highly enriched environments can interfere with the realisation of a person intelligence, regardless of their heredity.
Bouchard and Mcgue (2003) meta-analysis
- Looked at genetic and environmental influences on human psychological differences.
- Including general intelligence or ‘g’.
- Looked at the findings of numerous studies, to build an overall picture of how genetic/environmental factors influence ‘g’.
The Burt Affair 1976
What was Burt accused of?
- Faking data
- Inventing research assistants
- Creating fictious co-authors for his papers
- Inadequately describing his data collection methods
- Incorrect use of statistical mathods
- Claims appear to be supported by Burt’s own private diaries/correspondence.
- But there is still some debate
WHY FAKE IT?
Burt believed that the genetic influence on intelligence justified the status quo of the class system in early 1900s Britain
- The intelligent, wealthy, upper class are rightfully where they belong.
- The working class, who are less intelligent, are best placed in low paid
positions. - This is the “natural order of things”.
- Burt’s personal views possibly led to the falsification of data, to support his beliefs/theory.
