We study the first differences w ( t ) $w(t)$ of the International Sunspot Number (ISSN) daily series for the time span 1850 – 2013. The one-day correlations ρ 1 between w ( t ) $w(t)$ and w ( t + 1 ) $w(t+1)$ are computed within four-year sliding windows and are found to shift from negative to positive values near the end of Cycle 17 ( ∼ 1945 ). They remain positive during the last Grand Maximum and until ∼ 2009 , when they fall to zero. We also identify a prominent regime change in ∼ 1915 , strengthening previous evidence of major anomalies in solar activity at this date. We test an autoregressive process of order 1 (AR(1)) as a model that can reproduce the high-frequency component of ISSN: we compute ρ 1 for this AR(1) process and find that it is negative. Positive values of ρ 1 are found only if the process involves positive correlation: this leads us to suggest that the births of successive spots are positively correlated during the last Grand Maximum.