Environment is extremely important to an individual's mental or emotional development. The question does not qualify "a person's" as being developmentally challenged, but it can be assumed that that is at least part of the underlying question.
A child, fully functional or disabled to a greater or lesser degree, who is raised in a dysfunctional family without any positive outside influences, whether from relatives, friends, teachers, or therapists, is likely to exhibit developmental problems. Children are heavily influenced by what they see and by what they hear within the environments in which they are most frequently immersed. Parents who argue frequently, or who consume excess amounts of alcohol or, even worse, abuse drugs, are statistically more likely to raise children who grow up to have the same addictions or temperamental afflications.
Conversely, children, even those with learning disorders or mental disabilities, will fare much better if surrounded by a functional, loving family, including extended family. Additionally, with the emphasis over the past 30 years on providing special assistance in public schools to such children, they are more likely to prosper and learn to function in society and, ultimately, to contribute to that society. The 1975 Individuals with Disabilities Education Act and the later Americans with Disabilities Act required public schools to provide whatever special assistance is needed to ensure disabled children receive the attention and resources they need to learn and to develop the ability to socialize.
Social support systems are essential for emotionally handicapped children to develop the ability to function as a "normal" human being. In most cases, such children exhibit learning disorders, and require extra attention by trained staff to learn to overcome their difficulties. Absent such a support system, especially with the scale of overcrowding currently common in the public school system, mentally or emotionally disadvantaged children are simply swallowed up by the system, including by overworked teachers struggling with inordinately large class sizes. For this reason, the social welfare provisions included in the above-mentioned laws are essential for the benefit of special-needs students.