nature and nurture of language development

Question 1

language development

Answer 1

• Language ability:
  
  • Appears in the first three years of life
  • Across different languages and cultures
  • Without need of explicit instructions
  • Innate ability
    · Early language skills are very important to childrens chances:
  • Childrens readiness for school (Roulstone et al 2011, Snowling et al 2011)
  • Qualification gained at school (Parsons and Schoon, 2011)
  • Earnings, literacy skills and mental health in adulthood (Parsons and Schoon 2011, Schoon et al 2010a, Schoon et al 2010b)

Question 2

language ability and disability

Answer 2

• the most widely used test by all speech pathologists is the clinical evaluation of language fundamentals (CELF): child understanding and use of language (receptive and expressive language skills) and compares this to their peers of the same age (percentile rank)

Question 3

nature and nurture of language development

Answer 3

• nature = genetic factors
• nurture = environmental factors

Question 4

evidence for the role of genetic factors

Answer 4

1. Twin studies
   
   2. KE family with members with language disability
   3. Comparative genomic studies
   4. Genetic association studies

Question 5

twin design - MZ and DZ twins

Answer 5

• monosygotic (MZ) - share 100% of genes = single egg fertilised by single sperm, then splits in two
• dizygotic (DZ) - share 50% of genes = separate eggs fertilised by separate sperm

Question 6

twin studies

Answer 6

• We can compare resemblance (similarity) of MZ and DZ twins on a specific trait (e.g., language ability) by looking at correlations.

Question 7

twin studies of language ability - language ability is a heritable trait

Answer 7

· At age 2 years, heritability for vocabulary = 26%, and for grammar = 52% (in boys) & 43% (for girls)
· At age 4, the heritability estimates were even higher (45%-68%) and further increased by age 6 (44%-92%) (Rice, 2020)
· Convergent evidence for moderate-to-high heritability was obtained at age 16 in a different sample (TEDS) (Dale et al, 2018)
· Language ability is not 100% heritable; environmental influences also play an important role

Question 8

twin studies of language ability - limitations

Answer 8

· Large sample sizes are required
· Population-based samples are preferred, to avoid overrepresentation of MZ pairs
· It can be difficult to identify twins
· Language phenotypes are often limited and lacking in metrics interpretable for comparisons to singleton age peers
· Possible twinning effect

Question 9

KE family and developmental verbal dyspraxia

Answer 9

• behavioural phenotype - for every test of speech and language that was administered, the group of affected family members was, on average, significantly impaired relative to the unaffected group (Hurst, 1990)
• neural phenotype - bilateral voxel-based morphometry (VBM) analyses showing (in colour) some of the regions in which affected KE family members have significantly reduced grey matter (Belton, 2003)

Question 10

the language gene?

Answer 10

• FOXP2 gene is the gene that is responsible for the speech and language disorder in the KE family (Lai et al, 2001)
• a single mutation in the KE family leads to a loss of function of one copy of the FOXP2 gene, and that the one copy that remains is insufficient for normal brain development (haploinsufficiency), leading to the speech and language disorder

Question 11

evolution of the FOXP2 gene

Answer 11

• a comparison of the FOXP2 gene of humans, other primates and other placental mammals shows that FOXP2 is among the most highly conserved 5% of proteins, indicating that it has a fundamental role in mammals (Enard et al, 2002)

Question 12

summary

Answer 12

· Both genetic and environmental influences are important for language development
· Genetic influences are not static: heritability of language ability increases during development
· Multiple genetic factors have been identified (e.g., FOXP2), suggesting that language ability is a multifactorial trait
· Genetic and environmental factors do not act in isolation, but co-act, to influence child language development