**1. Introduction**

We need rapid and effective climate action to reduce global greenhouse gas (GHG) emissions and avoid catastrophic climate change. Annual emissions must decrease to close to half of their 2010 levels by 2030, and reach net-zero by 2050 to increase the probability of limiting temperature changes to 1.5 ◦C above preindustrial levels [1]. Yet there are some socio-demographic trends that may make it more difficult to achieve this.

One such trend is the shrinking of household sizes globally. Together with the rise in global emissions, the number of households has also been increasing, outpacing population growth. Several studies have shown that there is a strong link between household size and per capita energy use and GHG emissions [2–6] in both developed and developing countries [7]. When individuals live together, there are "economies of scale"—people tend to share appliances, tools and equipment, cook together and heat and cool common living spaces. These acts of sharing allow for the per capita energy use to diminish with rising household size. Thus, as households get smaller, the extent of sharing within

households reduces, while the per capita energy use and emissions rise. Some domains, such as energy consumption for heating, cooling and lighting, show substantially higher potential for household economies of scale [3,4,8] compared to others such as transport, clothing, and services [2,4,8]. There may also be social benefits associated with shared living and larger household sizes, as they tend to counteract trends of isolation and loneliness and build stronger communities [9,10]. Furthermore, recent research shows that members of grassroots initiatives, which may involve communal living such as eco-villages and Transition towns, manage to reconcile lower carbon footprints and less materialistic living with higher life satisfaction [11,12]. Recognizing the important role of household economies of scale and their social and environmental implications, researchers have advocated policies and initiatives that encourage larger households and sharing within and across households [13].

Yet, the majority of research evidence focusing on the role of household size for consumption-based energy and GHG emissions is restricted to single country studies. A notable exception is a comparative multivariate analysis of household energy requirements in Australia, Brazil, Denmark, India and Japan conducted by Lenzen and colleagues dating from 2006 [14]. There is a lack of up to date comparative studies between countries [5], examining these trends in a broader context and discussing the potential contextual differences across countries. An up-to-date country comparative perspective is important from a policy perspective: is advice on supporting sharing within and across household equally valid across all EU countries, or do these strategies need to vary and be adjusted to different contexts?

Furthermore, average household sizes differ between rural and urban areas and opportunities to share may also vary with urban-rural context [15,16]. Yet, studies that examine the interaction between household size and population density in a country-comparative setting are lacking. This article addresses this gap, analyzing the role of household size and its interaction with urban-rural typology across EU countries.

Our main finding is that household economies of scale vary substantially across consumption categories, urban and rural typology and EU countries. High household economies of scale are noted for home energy, real estate services, and miscellaneous services such as waste treatment and water supply; yet, some of the weakest household economies of scale occur in high carbon domains such as transport. Furthermore, Northern and Central European states are more likely to report strong household economies of scale—particularly in sparsely populated areas—compared to Southern and Eastern European countries. We discuss possible reasons for these patterns, as well as policy strategies to encourage sharing within and between households to contribute to climate change mitigation.
