Nature-Nurture Methods

If someone who was adopted as a child had been raised by their biological parents, would they have turned out to be different? What if they had different adoptive parents? Questions like these are part of the nature vs nurture debate. Nature argues that behaviours are innate, whereas nurture suggests the environment plays a role in behaviour development. What nature-nurture methods are there to investigate the nature-nurture debate, then?

• We will explore the nature-nurture debate by examining the various nature-nurture methods.
• First, we will take a look at what nature vs nurture psychology is and some nature vs nurture examples in research.
• We will learn about the methods psychologists use in nurture and nature, focusing on nature psychology theories such as twin and heritability studies and nurture psychology theories such as adoption studies.
• To finish off, to help you understand the topic, we will look at nature vs nurture in the context of cognitive development.

Fig. 1 - Various methods can be used to study nature versus nurture debates.

Nature vs Nurture: Psychology

The nature-nurture debate concerns the origins of our traits. The nature approach traditionally argues that biological factors like genes and brain structure determine our traits (including behaviour, development, cognition, or illnesses). While the nurture approach points to the environmental factors that shape who we are.

The debate shifted from black-and-white explanations to exploring how biological and environmental factors interact in recent years.

Nature vs Nurture: Examples

The Warrior Gene (MAOA) gene inhibits (lowers) aggression; people with low MAOA activity tend to act more aggressively when provoked. Young men who experienced severe trauma in their early years are likelier to engage in antisocial behaviour than those who didn't. However, the effect was modulated by the "warrior gene" activity.

Men who experienced trauma AND had low gene activity showed greater antisocial outcomes (Byrd & Manuck, 2014).

Schizophrenia was thought to be due to a single 'schizogene'; however, recent research has found it is actually a polygenic disorder that causes a predisposition to developing schizophrenia. The person is vulnerable to environmental triggers or stressors, increasing their chances of developing the disorder. This is known as the diathesis-stress model.

Methods used by Psychologists to Study Nature vs Nurture

The science of nature vs nurture influences is called behavioural genetics. Behavioural genetics investigates how individuals vary in traits and how much genetics or the environment account for this variation. The main methods of study in this field are family studies.

Fig. 2 - Family studies are great opportunities to study the nature versus nurture debate.

Nurture Psychology Theories: Adoption studies

Adoption Studies investigate if adopted children raised by the adopted family tend to share more traits with their biological or family that raised them. Therefore, adoption studies examine the impact of the environment alone on one's characteristics. If the behaviour of adopted children correlates more with their adopted relatives, the behaviour is likely due to nurture.

However, if, despite being raised apart from their biological parents, their behaviour correlates more with them, it is likely due to genes (nature). Limitations of adoption studies include:

• Adoptions are relatively rare and hard to study.
• Involving the biological family in adoption studies can be unethical if they don't wish to reunite.
• Adoption studies assume that adoptees are placed in a different environment, while children are often adopted into families that resemble their own.
• Adoption studies rely on correlational data; causality can't be inferred.

Nature Psychology Theories: Twin studies

Twin studies examine similarities between monozygotic and dizygotic twins. Monozygotic (MZ) twins share 100% of their genes, and dizygotic (DZ) twins share 50% of their genetic material. Both MZ and DZ twins also tend to share largely the same environment and upbringing, therefore:

• If a certain behaviour is more commonly shared between MZ twins but less likely to be shared by DZ twins, we can conclude it's more heritable.
• If the variation in IQ is similar between MZ and DZ twins, it's likely to be caused by environmental factors rather than genes.

Limitations of twin studies include:

• Twins are not representative of the non-twin population; growing up a twin is unusual and can attract different experiences and expectations compared to most people.
• Twin studies assume MZ twins are more similar than DZ twins because they share more genetic material. Factors other than genetics can explain greater similarities between MZ twins. MZ twins are always the same sex and look very similar. While this is not always the case for DZ twins, therefore MZ twins are likely to be treated more similarly than DZ twins.
• Twin studies assume both MZ and DZ twins share 100% of their "nurture", so their environment grows up. Yet, siblings within the same family may have quite different experiences growing up, for example, due to peer influences.
• Heritability measures genetic influence on a population level and only describes a particular population at a specific time.
• Twin studies rely on correlational data; causality can't be inferred.

Nature Psychology Theories: Assessing Heritability

Twin studies use concordance rates to assess heritability. Data from Monozygotic and Dizygotic twins are collected and compared to calculate the probability of developing the same trait for related individuals. High concordance rates point to a strong genetic influence or strong heritability.

A 100% concordance means a trait is always shared between individuals with the same set of genes (MZ twins). Significantly higher concordance rates in MZ twins than in DZ twins suggest a high degree of heritability.

A heritability of 0 indicates that genes do not influence the trait, and a heritability of 1 indicates that genes completely determine the trait.

Nature vs Nurture: Cognitive Development

The debate of nature vs nurture in cognitive development can be seen in the Minnesota twin study. They examined the heritability of intelligence, personality, and other traits, using MZ twins and comparing their environments.

The Minnesota Twin Study

Bouchard et al. (1990) conducted a study to examine the heritability of intelligence, personality, interests, and attitudes. Bouchard compared the personalities and cognitive abilities of MZ twins separated soon after birth to MZ twins that grew up together.

The sample consisted of over a hundred pairs of twins recruited from different countries. Participants were, on average, 41 years old at the time of testing. Bouchard used multiple methods to assess their personalities and cognitive abilities.

Findings of the Minnesota Twin Study

Overall, twins reared apart were just as similar in terms of personality, attitudes, occupational, and leisure interests as twins reared together, indicating a high degree of heritability of those traits. It was concluded that genes strongly impact behaviour and account for 70% of the variance in intelligence.

Fig. 3 - Overestimating genetic influence neglects other factors influencing behaviour.

Limitations of the Minnesota Twin Study

The heritability estimates from this study were likely overestimated since they assume any similarities between twins to be ONLY due to nature, while it's likely similar environments also influenced them.

1. Adoptee placement: twins tend to be adopted into similar families regarding their socioeconomic background and other key characteristics. Twins reared apart were still of the same sex, grew up in the same culture simultaneously, and were, therefore, likely to encounter similar environmental influences and opportunities.
2. To assess concordance rates and heritability, the similarities between MZ reared apart should be compared to DZ twins reared apart (control group). Bouchard and colleagues initially took measurements from such a control group but decided to remove this data. They made heritability estimates directly from the correlation of IQ between MZ twins reared apart, which questions the validity of their results.
3. Conflict of interest - the study was funded by an organisation supporting racism and eugenics. The biological determinism supported by twin studies can have harmful effects on society and has been used to support racism and segregation.