This study sought to compare the role of family functioning in self-worth and happiness of slow-paced individuals with Down syndrome in Tehran and Baku. The present research had a causal-comparative (ex post facto) design. The research population was comprised of adults with Down syndrome some of whom had the Alzheimer’s disease. Purposive sampling method was utilized and 40 adults with Down syndrome in Baku and 40 individuals with Down syndrome in Tehran were selected. The sample, thus, included those who suffered from Down syndrome with Alzheimer and those suffering from Down syndrome without this disease. The instruments included NEO-FFI (Costa & McCrae, 1992) and Oxford Happiness Inventory (Argyle et al., 1989). Results of MANOVA showed that there was a significant difference between the two groups as to the personality traits and happiness. The mean of personality traits including conscientiousness, openness to experience, agreeableness and extraversion was higher in Baku than Tehran, except for neuroticism which was higher in Tehran. These differences may be due to the special educational methods in Down syndrome centres in Baku, and the fact that adults with Down syndrome are safe and free in Baku. Therefore, implementation of new changes in the current educational and caregiving systems in Tehran is highly recommended.