nature and nurture of language development

key aspects of language ability

• appears in the first three year of life

• occurs across different languages and cultures

• occurs without need of explicit instruction (innate ability)

why are early language skills important to children’s chances?

• affects children’s readiness for school

• affects qualifications gained at school

• affects earnings, literacy skills, and mental health in adulthood

what is the most widely used test to assess children’s understanding and use of language?

clinical evaluation of language fundamentals (CELF)

what factors can affect language development?

• nature (genetic)

• nurture (environmental)

what evidence is there for the role of genetic factors in language development?

• twin studies

• KE family with members with language disability

• cognitive genomic studies

• genetic association studies

how can twins studies be used to investigate language ability?

we can compare resemblance of MZ and DZ twins on a specific trait by looking at correlations (heritability = 2(rMZ - rDZ))

is language ability a heritable trait?

• yes there is convergent evidence for moderate-to-high heritability was obtained at age 16 (Dale et al., 2018)

• language ability is not 100% heritable

what are the limitations of using twins studies to investigate language ability?

• large sample sizes are required

• population-based samples are preferred to avoid overrepresentation of MZ pairs

• can be difficult to identify twins

• language phenotypes are often limited and lack metrics interpretable for comparisons to singleton age peers

• possible twinning effect

affects of the behavioural phenotype of the KE family

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 et al., 1990)

affects of the neural phenotype of the KE family

bilateral VBM analyses show significantly reduced grey matter in some brain regions (Belton et al., 2003)

what is the gene that is responsible for the speech and language disorder in the KE family?

FOXP2 (a single mutation leads to a loss of function of one copy of the gene and the other copy that remains is insufficient for normal brain development (haploinsufficiency) leading to the speech and language disorder)

what indicates that the FOXP2 gene has a fundamental role in mammals?

comparison between humans, other primates, and other placental mammals shows that it is among the most highly conserved 5% of proteins (Enard et al., 2002)

what is a monogenic trait?

caused by a single mutation

how are genetic association studies used to show the role of genetic factors in language development?

• compare a group of cases (i.e. non-related people who share the same trait) to a group of controls (i.e. non-related people who do not have this trait)

• observe the genetic variance that is of a higher frequency in the group of cases

what is a multifactorial trait?

caused/influenced by multiple genetic and environmental factors