Polycystic ovary syndrome (PCOS) is a common and complex condition underpinned by insulin resistance. Yet the aetiology of PCOS remain elusive and the interaction of insulin resistance with various intrinsic and extrinsic factors are not well understood. Therefore, we aimed to undertake a systematic review and meta-analysis to generate novel data addressing the question: is insulin resistance intrinsic to PCOS? Specifically, we aimed to investigate the effects of body mass index (BMI), age and diagnostic criteria on insulin sensitivity measured by the gold standard euglycaemic-hyperinsulinaemic clamp in women with and without PCOS. The systematic review was conducted according to the principles of the Cochrane Collaboration and a series of meta-analyses using mixed modelling and standardised magnitude based inferences was performed. The methodology used adheres to the PRISMA checklist. The search returned 4,371 articles; of these, 25 articles met the inclusion criteria and were included in the meta-analysis. Overall insulin sensitivity was lower in women with PCOS compared to controls (27%, large effect). This lower insulin sensitivity was independent of BMI but BMI also independently exacerbated insulin sensitivity in PCOS and had a greater impact than it did in controls. Age also adversely influenced insulin sensitivity and had a greater impact in women with PCOS, but the effect of age became negligible when adjusted for BMI and diagnostic criteria. PCOS diagnostic criteria had a small effect on insulin sensitivity, with women diagnosed with the original National Institute of Health criteria demonstrating lower insulin sensitivity compared to women diagnosed with the Rotterdam criteria. We extend prior knowledge by confirming that insulin resistance is intrinsic to PCOS and highlight the potential importance of targeted treatment, including exercise and insulin sensitising medication, to assist in the management of PCOS.