Net corporate and other expenses were down to $258 million for the year, a $55 million improvement versus the prior year. We have two moving parts here. One, we moved $42 million of expenses above the line within our core underwriting result which supported an improvement, but on the other end, we had $38 million of one-off expenses in relation to restructuring costs and transaction costs. Transaction costs of $8 million were in relation to the 13D process and lost portfolio transfer, whilst restructuring costs were $30 million for the year as we accelerated our cost savings program. Other notable items impacting net income during the period included $59 million loss from mark to market on liability classified capital instruments, $35 million foreign exchange loss and $101 million tax benefit from the creation of a deferred tax asset as a result of the changes to the Bermuda income tax rules.
Moving to Slide 14, I’ll talk briefly about fourth quarter financials. Overall, it was a strong quarter with all three earnings engines positively contributing to net income and up year on year. For the underwriting result, this marked the fifth consecutive quarter of positive income since we committed to building a culture driven by strong underwriting. Our core underwriting profits increased 19% to $37 million with the combined ratio down 1.4 points to 93.4%. This includes 2.9 points of short-term incentives for staff, which when excluded would equate to a 90.5% combined ratio for the core business compared to 94.8% in 2022 or improvement of 4.3 points. Gross premiums written decreased 3% quarter on quarter in our core business. Top line growth was impacted by reductions in premiums in the reinsurance segment where premiums are down $49 million compared to the fourth quarter last year.
This was partially offset by insurance and service premiums, which increased by $26 million or 6%. Core MGA revenues increased by 21% to $56 million compared to the prior quarter and were driven mainly by growth from Arcadian and IMG. Margins increased — improved by 16.7 points to a strong 22% while we grew our MGA net service fee income till $12 million for the quarter. The total investment result for the quarter was strong at $65 million. This was driven by $78 million of net investment income, which is up by $27 million compared to the prior quarter as the derisked portfolio continues to benefit from rate increases. Unrealized and realized losses including from related party investment funds were $13 million. Net income of $94 million was a significant improvement versus the $27 million lost during the prior year quarter.
Other items impacting income included a $6 million restructuring charge, $19 million of foreign exchange losses and the previously mentioned one-time deferred tax benefit of $101 million related to Bermuda’s new income tax law. This quarter also includes a $30 million increase to our reserve margin within the corporate segment. As Scott mentioned earlier, this has been done from a position of strength on the back of strong performance and we look to improve the quality of our balance sheet. Common shareholders’ equity grew 13% during the quarter, supported by the mark to market movements on fixed income investments and growth in net income. Adjusting for AOCI, common shareholders equity growth was 6% in the quarter. Moving to Slide 15 and focus on premium trends including 1/1 renewals.
During 2023, we continued to take actions to improve the profitability of the book with additional actions during the second half of ’23 related to cyber and workers compensation segment. We believe these actions will impact our underlying premium growth during 2024, which will be driven by positive rates across our portfolio and volume growth in areas like North American program business, Accident & Health and International, which we are actively looking to grow. Rating trends in Q4 have remained broadly similar to the first nine months of 2023. Less than 10% of our overall book gets renewed in Q4, excluding North American program business, which has experienced average rate increases of around 7%. North American program business saw 6% rate increases during Q4, excluding cyber and workers compensation, which have both been under pressure and where we have taken portfolio actions to manage the profitability of our book.
Moving to the topic of renewals. At the January 2024 renewal period, we experienced positive rate increases across the majority of the business with an average rate change at around 3% across our reinsurance portfolio. This was mainly driven by US casualty and US property business. Overall, the renewals were orderly and in line with our expectations. Next, Slide 16 shows the year-to-date change in combined ratio for our core business and breaks the movements into individual subcomponents. Our portfolio actions have significantly improved the underwriting profitability with the combined ratio for our core business being 10.4 points better year over year on a like for like basis. Our headline combined ratio of 89.1% has benefitted from 4.6 percentage points of reserve releases linked to the LPT transaction.
However, the expense reallocation of $42 million results in around a 2 percentage point drag. Adjusting for these two and the short-term incentives previously mentioned, results on a like-for-like combined ratio of 91.2% compared to 101.6% in 2022. The core attritional loss ratio is marginally better at 64% or 1 percentage points up on the previous year and is partly impacted from mixed changes between insurance and services and the reinsurance segment and also from the large losses in the International business. The mixed changes resulted in better profit commissions which are captured in the acquisition cost ratio and has resulted in 1.4 points of improvement. Looking at both of the moving parts together, results in a net improvement of 0.4 points year on year.
Slide 17 and 18, we look at the investment portfolio and investment result. We have made clear progress during the year as we delivered a strong net investment income figure, increased our overall asset duration to 2.8 years from 1.8 years at Q4 2022 and locked in attractive reinvestment yield in excess of 4.5% on our investments during the year. We rotated our portfolio throughout 2023, investing over $1.8 billion as we increased our exposures to corporates and asset-backed securities. Portfolio rotation and higher rates have supported our net investment income during 2023 and we expect trend to continue as we aim to deliver strong and net investment income between $250 million to $265 million during 2024. Overall, our investment strategy remains unchanged and focused on maintaining a high-quality fixed income portfolio.
