This research addresses the global imbalance in regional development between coastal and interior areas, a key concern of the UN's Sustainable Development Goals (SDGs), particularly SDG 10, which aims to reduce inequality. Coastal areas, due to geographical advantages, have historically attracted industries, capital, and infrastructure, leading to higher economic growth compared to lagging interior areas. However, globalization is showing a shift of economic factors towards interior regions, suggesting a potential trend towards more balanced development. This study aims to identify patterns in this development balance trend between global coastal and interior areas, providing policy direction for regional management and sustainable development. The study considers the crucial interplay between population and economic factors, acknowledging their mutual influence on spatial development and regional disparities. Existing research presents conflicting views on the synchronicity of population and economic shifts, with some suggesting a temporal mismatch and others proposing a consistent spatial distribution for balanced development. This study addresses this gap by investigating the geographical spatial evolution of population mobility within coastal and interior areas, examining the impact of their dynamic spatial distribution on economic patterns, and determining whether the coastal-interior gap is narrowing or widening.

The study draws upon Friedman's core-periphery theory and synthesizes existing research on population mobility and economic development. It cites Mellinger et al. (2000) on the concentration of population and economic output in coastal areas, highlighting the contrasting developmental trajectories of coastal and interior regions. It reviews the contributions of macro-economists emphasizing the influence of population variables on economic fluctuations (Fan et al. 2022; Hao et al. 2022; Hugo, 2011), and Smith (1999)'s assertion that population mobility is both a cause and effect of economic development. The interdependency between population and economic factors in spatial development is also acknowledged (Razin and Sadka, 1994). The study notes the positive correlation between economic development and population growth (Lee, 2003; Marshall, 1890), while acknowledging ongoing debates about the synchronicity of population and economic factor movements. It reviews Ravenstein's (1889) work on population mobility and economic motives, followed by classical and neoclassical migration theories emphasizing the role of wealth disparities in influencing population movements from low- to high-income areas (Kuznets, 1968; Sassen, 2000). Finally, it highlights the contrasting viewpoints regarding the spatial synchronicity of population and economic changes, with some research suggesting a temporal mismatch (Cai et al. 2009; Deng et al. 2021), while others advocate for consistent spatial distribution as a marker of balanced development (Fan, 2004; Liu and Zhang, 2022).

The study employs a global perspective, dividing the world's land area into two sub-regions: 'near regions' (within 100 km of the coast) and 'far regions' (over 100 km from the coast). Data sources include WorldPop Hub population data (combining census, survey, satellite, and mobile data), and World Bank GDP data. Key variables analyzed include: Regional Population Proportion (RPP), Regional Economic Proportion (REP), Per Capita GDP (PGDP), Population Density (PD), Economic Density (ED), and Population-Economic Density (PED). The study uses a panel threshold regression model to investigate the nonlinear relationship between population mobility and economic transfer, considering per capita income level (pg) and PED as threshold variables. The model tests for single, double, and triple thresholds to identify significant differences in parameter estimates across different ranges of threshold variables. The analysis is conducted at both global and intercontinental scales, allowing for comparisons across different continents and economic development stages.

The study reveals significant differences in population concentration and economic patterns between global coastal and interior areas. From 2000 to 2018, coastal areas (18.43% of land area) experienced a population increase from 2.917 billion to 3.661 billion, while interior areas (81.57% of land area) increased from 2.951 billion to 3.815 billion. Coastal areas showed significantly higher population and economic densities, approximately 4.3 and 8.3 times higher than interior areas, respectively. Despite this, the study found that the population proportion in coastal areas slightly decreased (from 49.71% to 48.97%), while the interior area proportion increased (from 50.29% to 51.03%), indicating a weak 'coastal remoteness' trend. However, this population shift lagged behind the economic shift, with economic factors showing a more pronounced movement towards interior areas. The per capita GDP gap between coastal and interior areas narrowed from 2.08 times to 1.78 times during the study period. Intercontinental analysis revealed heterogeneity, with four continents (North America, South America, Oceania, and Europe) showing slight convergence of regional disparities, Africa showing an expansion of disparities, and Asia showing a significant contraction of disparities. Different intercontinental population-economic mobility patterns were also identified: population and economy landward, population landward-economy seaward, and economy landward-population seaward. Threshold regression analysis revealed that in coastal areas, population mobility's impact on wealth transfer is positive when per capita income exceeds a threshold (USD 3942), while in interior areas, this impact is consistently positive with increasing per capita income. Population-economic density also showed threshold effects, indicating that high density hinders wealth accumulation in both coastal and interior areas.

The findings challenge the assumption that a widening gap between coastal and interior areas is inevitable. While absolute balance might be unrealistic, the study demonstrates that the per capita income gap can be narrowed, promoting balanced development even with asynchronous population and economic factor movements. The 'coastal remoteness' trend, combined with the narrowing GDP gap, suggests that globalization is not necessarily exacerbating regional inequality. The different intercontinental patterns highlight the importance of considering economic development stage when analyzing population-economic interactions. The lag in population movement compared to economic shifts is attributed to factors such as changing labor requirements across industrialization stages, the time required for population decision-making, and the inherent inertia of population settlement patterns. The unexpected finding of higher per capita GDP in North American interior areas compared to coastal areas is attributed to the concentration of mature manufacturing industries and efficient transportation networks in the interior.

The study provides novel insights into the dynamic interplay between population mobility and economic patterns in global coastal and interior areas. It demonstrates that while regional development imbalances persist, globalization can lead to a convergence of per capita income gaps. Policy implications include a shift in focus towards balanced development strategies for both coastal and interior areas, recognizing the potential of interior regions for future investment and migration. Future research should explore the role of additional factors influencing population mobility, such as climate change, social welfare, and real estate prices.

The study's limitations include its focus solely on population proportion changes as a measure of population mobility, neglecting other contributing factors such as climate change, social welfare, and real estate prices. Further research could explore these factors in more detail. Additionally, the study relies on existing data sources, which may have inherent limitations regarding data accuracy and coverage.