Frustrated by the morass that is the study of human kinship systems, evolutionarily-minded researchers have recently adopted a pragmatic stance, replacing “matrilineal kinship” with biologically-inspired labels, such as “female-biased/based/centred kinship”. In turn, this is expedient to the adoption of hypotheses, data, and analyses informed by the study of kinship systems in other species. Bucking the trend, I draw on insights from anthropology to outline a framework for the study of kinship systems in cross-specific perspective. Specifically, I reframe lineal kinship organization in terms of biases in investment towards lineally related kin — that is, individuals related through females only, if the bias is matrilineal, and individuals related through males only, if the bias is patrilineal. In a given population, investment is effectively restricted to the offspring of females in one case, and to the offspring of males in the other. This is distinct from a bias in investment towards daughters and towards sons, respectively. Overall, I show that bridging the divide between the study of human and non-human kinship systems requires precision in the definition of key terms and concepts — more so than novel hypotheses, more data, and fancier analyses.