This Special Issue of Applied Psycholinguistics contains a set of articles on research related to the genetics of developmental language disorders. The guest editors, Mabel Rice and Steven Warren, have assembled cross-disciplinary perspectives on the topic. In doing so, they highlight essential questions concerning the biological mechanism of innateness, the contributions of environment, the measurement of complex behavior, the nature of genetic structure, and the acquisition of language by various groups of young children. The intersection of these questions, one with another, provides a unique contribution of both theoretical and practical consequence for the field of psycholinguistics.

The nature and causes of language disorders are research topics familiar to the readership of Applied Psycholinguistics. Current inquiries place these topics in a dynamic and sprawling multidisciplinary context manifested through investigations of linguistic acquisition, cognitive development, genetics, neurocortical processes, cognitive neurolinguistics, behavioral phenotypes, and language intervention. This list is not complete by any means, but it does suggest the wide front of the current search for better knowledge about what causes language disorders and the dimensions of language acquisition that are affected. Although there is considerable momentum underway in current inquiries, the full potential for scientific advancement is hampered by fragmentation in the field, in part attributable to partitioning by diagnostic categories of affectedness. Investigators (and their funding sources) often focus on a particular clinical group, such as specific language impairment (SLI), autism/autism spectrum disorders (ASD), Williams syndrome (WS), Down syndrome (DS), or fragile X syndrome (FXS). Although this is not exclusively the case, it is relatively difficult to carry out comparative studies across the multiple clinical conditions in which language disorders appear and to monitor developments across such a wide front of investigation. The consequence is that findings are distributed across different publication outlets and different groups of scholars, a situation that can limit our appreciation of the ways in which language disorders are manifest, and the identification of common etiological factors.

Language deficits occur in a variety of developmental disorders including autism spectrum disorders, Down syndrome, fragile X syndrome, specific language impairment, and Williams syndrome. This paper describes the specific pattern of linguistic deficits in each of these disorders in terms of speech production, semantic, and syntactic abilities as well as the relationship between cognitive and linguistic skills and the presence of a deviant or delayed pattern of development. In the spirit of synthesis across diverse literatures, preliminary comparisons among the language profiles of these disorders are made. The full picture, however, is incomplete given the current state of the literature, which tends to focus on the analysis of a narrow range of linguistic phenomena within a single disorder. The field is in need of research that systematically compares these disorders and leads to detailed descriptions of linguistic phenotypes of each disorder.

This paper focuses on designing studies that will compare children with developmental language disorders (DLD) drawn from several syndromes in which there are primary impairments in the acquisition of language. This kind of research can be used to address four key questions: (a) What are the developing language phenotypes that characterize specific disorders? (b) What factors are key precursors and predictors of language acquisition in DLD? (c) What are the genes that contribute to DLD in different syndromes? (d) What environmental factors influence the trajectories of language development in DLD? Several design issues are discussed including an overall study design, subject selection and recruitment, matching and comparisons across groups, and methodologies. A number of important challenges to the design and implementation of these kinds of studies are presented in the final section of the paper.

Accurate phenotypic description is critical for the success of studies of the genetic basis for developmental language disorders. An important purpose of such a phenotypic description is to differentiate the language and associated cognitive profiles of syndromes or other developmental language disorders with diverse genotypes. In this paper we consider six measurement issues relevant to genotype/phenotype research and profiling: (a) Who is the target population? (b) What is the “ideal” measure of a single component of language? (c) What is the “ideal” measure(s) for quantifying the language (or language and cognitive) profile for a particular syndrome or disorder? (d) What are the special measurement issues for infants and young children? (e) How do we develop a profile? (f) What are the unresolved issues?

Neurocognitive studies can approach gene-based developmental language impairments from two angles, which are complementary and ideally combined in a research program. One approach aims at an optimal phenotypic description of a disorder and from there proceeds to a biological and developmental understanding. Complementary to such a top-down approach, a bottom-up perspective will primarily focus on potential etiological pathways and attempt to explain complex outcome phenotypes in terms of elementary developmental disturbances. My paper is dedicated to this latter approach. I argue that in behaviorally defined disorders (such as specific language impairment or autism) shared genetic risk and common etiology can at best be expected for specific aspects of language deficit and that such shared etiology will only apply to subtypes of these disorders. One reason for this skepticism is that the emerging language system in children can be affected in many different ways via more elementary sensory, perceptual, cognitive, and motor impairments. Neurocognitive research on developmental language disorders relies on an understanding of such potential elementary disturbances before it can confidently proceed to the study of complex linguistic impairments.

There is a good deal of interest in the application of neurocognitive techniques to investigate the underpinnings of developmental language impairments (DLIs). Electrophysiological techniques such as electroencephalography and magnetoencephalography offer the promise of the ability to track brain activity with precision in time and space. This article describes a number of findings from studies of normal adults and children that are relevant to neurocognitive studies of developmental language impairments and outlines a series of challenges that should be met in order for electrophysiological measures to realize their promise.

Knowledge of the environmental and genetic etiologies of complex cognitive disorders can guide strategies for diagnosis, prevention, and therapy, but disentangling the various causes can be very challenging. Two basic approaches can be used in identifying genetic factors, a top-down approach, in which phenotypic descriptions are used to discover genes that influence those phenotypes, and a bottom-up approach, comparing the phenotypic effects of genes that are known to cause syndromes that include cognitive disabilities. Thorough characterization of phenotypes throughout development is critical to both of these methods. These strategies have been applied to the characterization of genetic etiologies for reading disability, language disorders, attention-deficit hyperactivity disorder, and autism.

Developmental disorders affecting speech and language are highly heritable, but very little is currently understood about the neuromolecular mechanisms that underlie these traits. Integration of data from diverse research areas, including linguistics, neuropsychology, neuroimaging, genetics, molecular neuroscience, developmental biology, and evolutionary anthropology, is becoming essential for unraveling the relevant pathways. Recent studies of the FOXP2 gene provide a case in point. Mutation of FOXP2 causes a rare form of speech and language disorder, and the gene appears to be a crucial regulator of embryonic development for several tissues. Molecular investigations of the central nervous system indicate that the gene may be involved in establishing and maintaining connectivity of corticostriatal and olivocerebellar circuits in mammals. Notably, it has been shown that FOXP2 was subject to positive selection in recent human evolution. Consideration of findings from multiple levels of analysis demonstrates that FOXP2 cannot be characterized as “the gene for speech,” but rather as one critical piece of a complex puzzle. This story gives a flavor of what is to come in this field and indicates that anyone expecting simple explanations of etiology or evolution should be prepared for some intriguing surprises.

This paper describes the next steps for establishing detailed, useful behavioral phenotypes and for moving forward in the study of the relationships among genes, the environment, and language disorders. Key issues are recognizing the continuum of language behaviors to accurately describe disorders and the spectrum of typical development. A coordinated plan with foundational studies that address definition, measurement, design, and analytical methods is needed. Researcher capacity must be increased, and a greater focus on communication across disciplines to successfully accomplish interdisciplinary research is required. Language is not static, even in disorders; thus, we must study change over time. It is necessary to think about behavioral phenotypes in terms of developmental trajectories. Finally, data sharing and ethical and legal issues must be addressed.