The research community has long known that having active primitive reflexes as a child or adult can cause problems like ADD/ ADHD, muscle and joint pain, reading problems, and even dyslexia. Here is some of the scientific literature that discusses primitive reflexes:
Reflex development and motor development are closely linked
A longitudinal study published in Developmental Study and Child Neurology in 1994 conducted with low-birthweight, preterm infants suggests that there is a strong relationship between primitive reflex patterns, automatic postural reactions, and motor development in the first year of life. It found that the prevalence of primitive reflex patterns, such as ATNR, predicted the prevalence of mature postural reflexes, such as the Landau and the righting reflexes. There is also a correlation between the integration of the TLR and the presence of the ATNR at one year of age as well as later achievement of motor milestones.
Mandich, M., Simons, C. J., Ritchie, S., Schmidt, D., & Mullen, M. (2008). Motor Development, Infantile Reactions And Postural Responses Of Preterm, At-Risk Infants. Developmental Medicine & Child Neurology,36(5), 397-405. doi:10.1111/j.1469-8749.1994.tb11865.x
ATNR, Plantar, and Moro Reflexes may be active in infants at risk for developmental delays
This 2015 literature review sought to find the connection between active primitive reflexes in children and their risk for developing Cerebral Palsy or experiencing developmental delays. It found that in early infancy, an active Moro and Plantar Grasp reflex correlated with the diagnosis of Cerebral Palsy, and after early infancy an active Moro and ATNR predicted developmental delays.
Hamer, E. G., & Hadders-Algra, M. (2016). Prognostic significance of neurological signs in high-risk infants - a systematic review. Developmental Medicine & Child Neurology,58, 53-60. doi:10.1111/dmcn.13051
The correlation between active primitive reflexes and ADHD
This 2013 study tested to what extent an active ATNR and STNR is related to ADHD symptoms in girls aged 8-11. Their methods found that "persisting primitive reflexes are closely linked to ADHD symtoms." View the full report here.
Bob, P., Konicarova, J., & Raboch, J. (2013). Persisting primitive reflexes in medication-naïve girls with attention-deficit and hyperactivity disorder. Neuropsychiatric Disease and Treatment,1457. doi:10.2147/ndt.s49343
The presence of active primitive reflexes in infants and their correlation with neuro-developmental abnormalities
For most primitive reflexes, retention of the reflex beyond the period when it should no longer be elicited suggests a pathologic process within the central nervous system. However, for certain primitive reflexes, such as the plantar grasp reflex, a negative response within the first months of life is suggestive of a neurological abnormality. From the results of one prospective and one retrospective study, it is clearly indicated that the absence of the plantar grasp reflex from 3 months of age and on correlates with the development of spastic cerebral palsy. The specific combination of presence or absence of specific primitive reflexes, postural reactions, or both may accurately predict a specific type of cerebral palsy or neuro-developmental abnormality.
Zafeiriou, D. I. (2000). Plantar grasp reflex in high-risk infants during the first year of life. Pediatric Neurology,22(1), 75-76. doi:10.1016/s0887-8994(99)00110-1