These are two paragraphs which i've selected from two of the best papers about biology of SP. they say genetics and environmental factors both affect SP. may be environmental factors plays a greater role:
1)Genetic factors vs. environmental factors
Genetically speaking, some animal models have shown social temperament to be breed-dependent. Kalin and Shelton (28) commented on the common ground shared by the novelty-aversive behavior of children with BI and the defensive behaviors of infant rhesus monkeys. (Interestingly, in infant rhesus monkeys the fear-related freezing and defensive barking have been shown to share common features, and were relieved by diazepam.) Despite genetic influence, animal models of behavior show that temperamental predisposition, and potentially SAD, is indeed affected by the rearing environment. In laboratories, postnatal handling of rat pups by humans increased maternal licking and grooming and arched-back nursing, while maternal separation reduced this behavior (29) to a level equivalent to that of non-handled pups (30). Furthermore, the offspring of non-handled pups showed greater fearfulness in unfamiliar surroundings (31). Pups experiencing maternal separation and their non-handled cohorts were also less likely than handled pups to feed in a novel environment. Moreover, handled pups spent significantly more time exploring novel environments and showed reduced startle response when compared with the other cohorts (31). (The neurobiologic mechanisms are discussed below.) To reinforce the discussion of environment vs. genetics, the concept of cross-fostering has been explored. BALBc mice are typically very fearful and wary of novel environments. However, once cross-fostered to C57 mothers, known for a twofold greater frequency of licking and grooming, the BALBc pups become significantly less fearful (31– 33). Another study showing the effect of early life stress was conducted in non-human primates. Infant bonnet macaques were subjected to differences in maternal stress induced by food availability. Infants raised by mothers exposed to variable foraging demands showed significantly more fearfulness and aversion to social interaction in adulthood than those infants raised with a reliable food supply (34, 35).
2)Genetics of Social Anxiety Disorder
Although there is increasing evidence that social anxiety disorder and its childhood variants, including behavioral inhibition and shyness, have a strong familial basis, the genetics of the disorder have not been adequately studied.
Several early studies (58, 59) established a familial link, but only for the generalized subtype (59). It was reported that if a proband has a diagnosis of social anxiety disorder, the percentage of first-degree relatives with the illness was 15%, which was greater than the 10% finding in subjects with agoraphobia and less than the 31% seen in subjects with simple phobia (49). Subsequently, a larger study (60) showed that the generalized subtype was markedly increased in frequency (approximately 10 times greater) among first-degree relatives of generalized social phobic probands. Another study (61) demonstrated that the children of patients with social anxiety disorder were at an increased risk of developing this disorder and other anxiety disorders.
***The low genetic concordance rates for social anxiety disorder in monozygotic twins (62) have suggested that genetics plays a limited role in its development.*** As we suggested for panic disorder (1), what appears to be inherited is a susceptibility to social anxiety, not the disorder itself. Although no systematic genetic linkage studies employing a genomic scan or search among candidate genes have been conducted for social anxiety disorder so far, such studies are underway for panic disorder (63) and OCD (64). Likewise, molecular genetic studies of candidate genes for the several neurotransmitter systems implicated in social anxiety, notably the serotonin transporter and dopamine receptor and their various subtypes, have allowed
for associations between specific genes and behavioral traits, such as harm avoidance and novelty seeking (65, 66)—characteristics relevant to the social anxiety disorder phenotype. Thus, genetic and family studies in social
anxiety disorder are still in their infancy but support longitudinal clinical data that are suggestive of links between childhood and adult variants of the disorder.