The most important concept in custody is "best interests". Advancing the best interests of the child is always supposed to be the goal of the court. So, it is important to understand the concept and understanding begins with history.
"Best interests" have been the guiding principle in Pennsylvania custody since 1813. Prior to 1813, the absolute "common law" rule was that children were the property of their father. In addition, mothers were not fully "persons" under the law. The pendulum was stuck all the way in favor of fathers.
But in the 1813 case of Commonwealth v. Addicks, the Pennsylvania Supreme Court awarded custody of two young daughters to their (adulterous) mother on the theory that it was best for the girls, given their age. Interestingly, the same Supreme Court then transferred custody of the girls back to their father, once the girls were old enough to understand their mother's adultery. Once again, the girls' "best interests" was the key.
The best interests standardeventually moved beyond caselaw and was codified by the Pennsylvania legislature in their custody act of 1895.
Ultimately, the best interests standard evolved into the longstanding "tender years" doctrine, which was an almost ironclad principle that children were best off with their mothers during their "tender years". By now, the pendulum had swung and stuck in favor of mothers.
Finally, in 1977, the Pennsylvanian Supreme Court in Spriggs v. Carson found that the "tender years" doctrine violated the constitutional principle of equality of the sexes. The
Spriggs court also warned lower courts to be "wary of deciding matters as sensitive as questions of custody by the invocation of presumptions.".
The Spriggs case officially eliminated the tender years doctrine. But the Supreme Court's general warning about presumptions went largely unheeded. Some courts continued to surreptitiously use the "tender years" doctrine. Other courts applied other presumptions.
For example, in 1982, the Superior Court rejected presumptions based on race or extramarital relationships.
In separate 2010 cases, the Superior Court rejected sexual orientation and a modified "tender years doctrine" as presumptions. But, to this day, some courts have applied an unofficial but de facto "presumption of shared physical custody" approach.
The new custody law explicitly eliminates almost all presumptions (except for those favoring parents against non-parents). And most courts now approach custody on a case-by-case basis. But it may be difficult to eliminate all unofficial presumptions. "Best interests" remains a broad, vague and easily manipulated doctrine.