Current issues in sibling detection

• Coresidency, observing infant-directed care, and resemblance are siblingship cues.
• Coresidence duration is a proxy for the tracking of shared parental investment.
• Factors such as emotional closeness also influence cooperative behavior.Over the past century, we have learned much about how humans learn to categorize individuals as close genetic relatives. In the late 1800s Edward Westermarck sparked psychological and anthropological research into the role coresidence duration plays as a cue to siblingship and as an important factor regulating sexual attraction. Since then, researchers have collected additional evidence that observations of maternal–infant investment and facial resemblance also serve as cues to siblingship. Here we review recent work on sibling detection and discuss future directions for research.