Inbreeding increases the chances of the expression of deleterious recessive alleles by increasing homozygosity and therefore has the potential to decrease the fitness of the offspring. With continuous inbreeding, genetic variation is lost and homozygosity is increased, enabling the expression of recessive deleterious alleles in homozygotes. The inbreeding coefficient, a term used to describe the degree of inbreeding in an individual, is an estimate of the percent of homozygous alleles in the overall genome.  The more biologically related the parents are, the greater the inbreeding coefficient (See Coefficient of Inbreeding ), since their genomes have many similarities already. This overall homozygosity becomes an issue when there are deleterious recessive alleles in the gene pool of the family.  By pairing chromosomes of similar genomes, the chance for these recessive alleles to pair and become homozygous greatly increases, leading to offspring with autosomal recessive disorders. 
Twin studies manipulate the environmental factors of behavior by examining if identical twins raised apart are different from twins raised together. Before the advancement of molecular genetics , twin studies were almost the only mode of investigation of genetic influences on personality. Heritability was estimated as twice the difference between the correlation for identical, or monozygotic, twins and that for fraternal, or dizygotic, twins. Early studies indicated that personality was fifty percent genetic. Current thinking holds that each individual picks and chooses from a range of stimuli and events largely on the basis of his genotype creating a unique set of experiences; basically meaning that people create their own environments.