Identical Twins and Intelligence 3

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Identical Twins and Intelligence 1 Identical Twins and Intelligence Student's Name Institutional Affiliation

Identical Twins and Intelligence 2 Identical Twins and Intelligence Identical twins are believed to share the same intelligence level since they originate from a single fertilized egg that splits into two embryos thus nearly sharing identical genetic makeup. The study of identical twins is essential in revealing the complex phenomenon in regard to nature versus nurture's influence on intelligence. While genetics significantly influence cognitive abilities, environmental factors also play a crucial role in determining the intelligence level of individuals. Research comparing the intelligence of identical twins raised in different environments in comparison to those raised together is beneficial in understanding the complex interplay between genes and external influences in shaping intellectual capabilities. Overall, the unique genetic similarity of identical twins provides valuable insights into the intricacies of intelligence development. Identical Twins Formation Identical twins, also known as monozygotic twins, occur when a single fertilized egg, also known as a zygote, splits into two separate embryos (Turrina et al., 2021). This often happens shortly after fertilization, typically within the first two weeks of pregnancy. The process of the zygote splitting results in two embryos that are genetically identical because they share the same set of chromosomes. The occurrence of identical twins begins with the fertilization of a single egg by a single sperm. This forms a zygote, which is a fertilized egg containing a complete set of chromosomes, half being from the mother and half from the father. Then follows cleavage where the zygote divides into a multicellular structure through a series of cell divisions. At around the fourth or fifth day after fertilization, blastocyst formation occurs in which the multicellular structure becomes a blastocyst. The blastocyst is a hollow ball of cells with an inner cell mass. Next, the inner cell mass of the blastocyst undergoes a process called embryonic

Identical Twins and Intelligence 3 disc splitting. This is the critical step where the cells separate into two distinct masses resulting in the formation of two distinct embryos within the same gestational sac. These two embryos share the same genetic material because they originated from the same zygote. The two embryos then implant into the uterine wall, and the pregnancy proceeds with two genetically identical individuals developing side by side (Beck et al., 2021). Intelligence Intelligence is a multifaceted and complex cognitive ability that encompasses various mental processes such as learning, reasoning, problem-solving, and adaptation to new situations. Intelligence entails the capacity of an individual to acquire and apply knowledge, understand abstract concepts, and engage in critical thinking (Hill et al., 2018). Intelligence is not a singular trait but rather a spectrum as individuals exhibit diverse intellectual strengths and weaknesses. The concept of intelligence has evolved over time, with psychologists proposing different theories such as the psychometric approach and multiple intelligences theory. While standardized intelligence quotient (IQ) tests have traditionally been used to measure intelligence, there is ongoing debate about the accuracy and inclusivity of such assessments. Moreover, cultural and contextual factors play a significant role in shaping how intelligence is perceived and expressed. Generally, intelligence is a dynamic and context-dependent quality that plays a crucial role in human adaptation, critical thinking, and problem-solving. Genetic Influence Identical twins originate from a single fertilized egg that splits into two embryos and thus share nearly identical DNA and genes. Due to this fact, identical twins genetically share the same intelligence level. Similarly, the heritability estimate for intelligence is approximately 50-80% (Fischbach & Niggeschmidt, 2021). Since identical twins come from the same egg and sperm,

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Identical Twins and Intelligence 4 they are bound to inherit the same intelligence from their parents. Additionally, the brain structure, neurotransmitter function, and synaptic connectivity which contribute to cognitive abilities are influenced by genetic factors. Research has also established that the difference in cognitive abilities including intelligence is significantly attributed to genetics. According to Procopio et al. (2022), approximately 50% of the variance of intelligence observed in human beings is attributed to inherited DNA differences. This implies that inherited genetics significantly influences intelligence thus identical twins who share the same genes are bound to share the same intelligence level. Environmental Influence Although identical twins share nearly 100% genetic makeup, the environment significantly influences their intelligence. While genes play a crucial role in shaping cognitive abilities, environmental factors contribute substantially to the development and expression of intelligence. This is the main reason why identical twins raised in distinct environments often experience variations in intelligence level. Educational opportunities, social interactions, family environment, and cultural stimuli significantly contribute to the differences in cognitive outcomes in identical twins. For instance, the majority of the identical twins living in an urban residential environment surrounded by green space were found to have higher intelligence levels than their twins living in rural and semi-urban areas (Bijnens et al., 2020). Additionally, foods such as oily fish, nuts, and berries improve brain function and memory thus increasing intelligence. Also, twins who were assigned to advanced education systems and good teachers attained higher academic achievements thus exhibiting higher intelligence levels than their counterpart twins who were exposed to poor teachers and

Identical Twins and Intelligence 5 underdeveloped education systems (Larsen et al., 2019). Generally, environment substantially contributes to the divergent level of intelligence among monozygotic twins. Conclusion The question of whether identical twins have the same level of intelligence is complex since it involves both genetic and environmental factors. Identical twins, also known as monozygotic twins, originate from a single fertilized egg that splits into two embryos. As a result, they share nearly identical genetic material. While genetics play a significant role in determining the intelligence of an individual, environmental factors also contribute. Identical twins may have similar genetic makeup but may still experience differences in their environments such as prenatal conditions, early childhood experiences, education, and social and economic interactions. These environmental factors can lead to variations in cognitive development and intelligence. As observed from research studies on identical twins, findings suggest that there is a genetic component to intelligence, with heritability estimates typically ranging from 50% to 70%. However, this does not mean that intelligence is entirely determined by genetics. Environmental influences, especially during critical periods of development, also play a crucial role. In conclusion, while identical twins share a high degree of genetic similarity, environmental factors can contribute to differences in their intelligence levels. The interplay between genetics and environment makes it challenging to predict or guarantee identical levels of intelligence between twins, even if they share the same genes.

Identical Twins and Intelligence 6 References Beck, J. J., Bruins, S., Mbarek, H., Davies, G. E., & Boomsma, D. I. (2021). Biology and genetics of dizygotic and monozygotic twinning. Twin and Higher-order Pregnancies , 31-50. https://doi.org/10.1007/978-3-030-47652-6_3 Bijnens, E. M., Derom, C., Thiery, E., Weyers, S., & Nawrot, T. S. (2020). Residential green space and child intelligence and behavior across urban, suburban, and rural areas in Belgium: A longitudinal birth cohort study of twins. PLOS Medicine , 17 (8), e1003213. https://doi.org/10.1371/journal.pmed.1003213 Fischbach, K., & Niggeschmidt, M. (2021). Heritability of intelligence: A clarification from a biological point of view (3rd ed.). Springer Nature. Hill, W. D., Marioni, R. E., Maghzian, O., Ritchie, S. J., Hagenaars, S. P., McIntosh, A. M., Gale, C. R., Davies, G., & Deary, I. J. (2018). A combined analysis of genetically correlated traits identifies 187 loci and a role for neurogenesis and myelination in intelligence. Molecular Psychiatry , 24 (2), 169- 181. https://doi.org/10.1038/s41380-017-0001-5 Larsen, S. A., Byrne, B., Little, C. W., Coventry, W. L., Ho, C. S., Olson, R. K., & Stevenson, A. (2019). Identical genes, unique environments: A qualitative exploration of persistent monozygotic-twin discordance in literacy and numeracy. Frontiers in Education , 4 . https://doi.org/10.3389/feduc.2019.00021 Procopio, F., Zhou, Q., Wang, Z., Gidziela, A., Rimfeld, K., Malanchini, M., & Plomin, R. (2022). The genetics of specific cognitive abilities. Intelligence , 95 , 101689. https://doi.org/10.1016/j.intell.2022.101689

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Identical Twins and Intelligence 7 Turrina, S., Bortoletto, E., Giannini, G., & De Leo, D. (2021). Monozygotic twins: Identical or distinguishable for science and law? Medicine, Science and the Law , 61 (1_suppl), 62- 66. https://doi.org/10.1177/0025802420922335

Identical Twins and Intelligence 3

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